Why does white light viewed through optical chopper appear purple/blue?

I looked at a light like the one shown here through a fidget spinner (I believe I was observing near where the "blades" meet the inner portion) and noticed that it appeared purple/blue, particularly when viewed for a prolonged period (I looked at the reflection of sunlight from snow and observed this as well.). I believe this is either an optical effect (the spectrum is convolved with the Fourier transform of the "chopping function or pulse" ) or has something to do with the eye/my perception.

So what's going on here?

I totally wasn't expecting to immediately duplicate this with a fidget spinner and the fluorescent light in my office. What a neat effect!

My technique: I hold the spinner quite close to my eye, so that it occupies nearly all of my field of view, and look through the spinning part at my ceiling light. If the spinner is very fast or very slow, I don't see any change in the hue of the light. However there's an intermediate rate where the back part of the fluorescent lamp takes on a patchy blue tint with colored edges. The tinted region is irregular and unpredictable in the same way as a pressure phosphene, if you're familiar with that better-documented effect, and has kind of the same "feel" to it.

Trying to photograph the effect just gives the rolling shutter problem, so that's no use. (No color change in the photo, either.)

At the rotational speed where the color change effect is the most pronounced, I estimate the frequency at which the spinner blocks nearly all of the light to be 10-20 Hz. (How? I let the thing hit a fingernail and listened to it tap; it sounds like thirty-second notes, four to a click, at a good presto tempo.) I've read elsewhere that the human eye can process images at 10-15 Hz. So some sort of biological stroboscopic effect, where the eye has just enough time to begin to respond to the reduced light before the light returns, seems plausible to me.

Physical basis of colors seen in Congo red-stained amyloid in polarized light

Amyloid stained by Congo red is traditionally said to show apple-green birefringence in polarized light, although in practice various colors may be seen between accurately crossed polarizing filters, called polarizer and analyzer. Other colors are seen as the polarizer and analyzer are uncrossed and sometimes when the slide is rotated. Previously, there has been no satisfactory explanation of these properties. Birefringence means that a material has two refractive indices, depending on its orientation in polarized light. Birefringence can change linearly polarized light to elliptically polarized, which allows light to pass a crossed analyzer. The birefringence of orientated Congo red varied with wavelength and was maximal near its absorption peak, changing from negative (slow axis of transmission perpendicular to smears or amyloid fibrils) on the shortwave side of the peak to positive (slow axis parallel) on the longwave side. This was explained by a property of any light-absorbing substance called anomalous dispersion of the refractive index around an absorption peak. Negative birefringence gave transmission of blue, positive gave yellow, and the mixture was perceived as green. This explains how green occurs in ideal conditions. Additional or strain birefringence in the optical system, such as in glass slides, partly or completely eliminated blue or yellow, giving yellow/green or yellow, and blue/green or blue, which are commonly seen in practice and in illustrations. With uncrossing of polarizer or analyzer, birefringent effects declined and dichroic effects appeared, giving progressive changes from green to red as the plane of polarization approached the absorbing axis and from green to colorless in the opposite way. This asymmetry of effects is useful to pathologists as a confirmation of amyloid. Rather than showing ‘apple-green birefringence in polarized light’ as often reported, Congo red-stained amyloid, when examined between crossed polarizer and analyzer, should more accurately be said to show anomalous colors.

Many pathologists use Congo red to make a diagnosis of amyloid and state the common opinion that in polarized light, Congo red-stained amyloid shows apple-green birefringence, sometimes called apple-green dichroism. Is this opinion correct? A cursory glance at published micrographs said to illustrate this color reveals that most show more than one color and that some do not even show green. In everyday practice, a diligent pathologist may have been dissuaded from a diagnosis of amyloid, or at least puzzled, if green was not the only color or if green was not seen. If the general opinion is correct despite these findings and illustrations, what is the explanation of the green color? If it is not correct, what is the explanation of the colors seen, and what is a better expression to use? Does an understanding of the mechanisms help the pathologist in practice or give an insight into the interaction between Congo red and amyloid?

No previous explanation of the mechanisms has been satisfactory, partly because of incomplete accounts of the properties of Congo red-stained amyloid. We investigated these using polarizing microscopy. 1, 2, 3 Because some principles of polarizing microscopy may not be widely known, they are given here to help understand this study.

Polarizing Microscopy

Polarization means that light waves vibrate in only one plane. One way to achieve this is by use of the polarizing filters available for microscopes, called a polarizer when inserted in the optical path between light source and specimen stage and an analyzer when inserted between the specimen stage and eyepieces. Polarization effects are generally identical if either filter is inserted on its own, or if either is rotated. If just a polarizer or an analyzer is inserted, without a specimen, the field appears bright, and the observer cannot detect that the light is polarized, even if the polarizing filter is rotated. If the other polarizing filter is also inserted, and one of them is rotated, the field now changes from light to dark, with maximal darkness when the planes of polarization of polarizer and analyzer are at right angles, and the filters are said to be crossed. When Congo red-stained amyloid is considered, this arrangement is what is usually meant by ‘polarized light,’ but a more accurate expression is examination of a specimen between crossed polarizer and analyzer. There are two observations to explain, namely, how Congo red-stained amyloid appears bright against a dark background under these conditions, and how it appears colored.

Absorption and Dichroism

Materials appear colored because they absorb light of certain wavelengths and their color is white lacking those wavelengths. The visible spectrum runs from violet at wavelengths of about 400–430 nm, through blue, 430–490 nm, blue/green, 490–510 nm, green, 510–570 nm, yellow, 570–600 nm, orange, 600–620 nm, to red, 620–700 nm or more. These wavelengths are approximate and the colors merge into each other. Red, such as the appearance of Congo red-stained amyloid in unpolarized light, is produced by the predominant absorption of blue/green or green, around 500–520 nm. Green is produced by the predominant absorption of both violet and red light, the absorption of which individually gives yellow and blue. Any green color can be matched by a suitable mixture of yellow and blue light. Absorption is measured with a spectrophotometer and is expressed as optical density, which has no units, and is given by

At the molecular level, light is only absorbed when it is polarized parallel to a light-absorbing atomic bond. If a material is orientated so that all these bonds are parallel, appropriate wavelengths will be maximally absorbed from white light which is polarized parallel to the bonds, and the material will have its deepest color. No wavelengths will be absorbed when the light is polarized perpendicularly to the bonds, and the material should appear colorless. If this orientated material is on the specimen stage of a microscope fitted with a polarizer or an analyzer, but not both, and either the stage or the polarizing filter is rotated, the appearance of the material should change from its deepest color to colorless. This property is called dichroism, which means that either a material has different amounts of absorption of light polarized in different planes, or the material varies from colored to colorless, depending on the plane of polarization. 4 The second definition follows from the first. To investigate dichroism, a soluble dye such as Congo red, which is randomly orientated in solution, can be orientated by crystallization, smearing of a drying drop on a glass slide, buffing of a dried drop, or binding to a material that itself has an orientated structure. ‘Apple-green dichroism’ of Congo red-stained amyloid is incorrect, because even if there is a green color, it is seen between crossed polarizer and analyzer, not when Congo red is examined with only one polarizing filter, which is the way to detect dichroism.

Strong absorption may make an orientated material appear bright against a dark background when examined between crossed polarizer and analyzer. When the material is at 45° to the plane of the polarizer, polarized light is resolved into two vectors at right angles, one parallel to the absorbing bonds and the other perpendicular. After leaving the material, the vectors recombine, and the absorption in one axis may have been intense enough to make the plane of linearly polarized light rotate toward the non-absorbing axis. This rotation may allow some light to pass the crossed analyzer, and the material appears bright. This mechanism has been suggested as an explanation of the brightness of Congo red-stained amyloid between crossed polarizer and analyzer. 5

Birefringence, Retardance, and Compensation

The mechanism usually said to be the explanation of this brightness is birefringence, but without further explanation of how birefringence makes something appear bright and colored. Birefringence means that a material has a range of refractive index that varies between two extremes, where

Substances such as some types of glass have random arrangement of molecules and are isotropic. The refractive index is the same whatever the plane of light transmitted. Other materials, such as unstained collagen bundles, have an orientated arrangement and are anisotropic. The velocity of transmission differs between light polarized in one plane, for instance, parallel to the long axis of bundles, and light polarized at right angles. These materials have two extremes of refractive index, of which the higher indicates the slow axis of transmission and the lower indicates the fast axis. Birefringence is a property that cannot be colored apple-green or any other color, although it may be the explanation of colors. The amount of birefringence, which has no units, is the difference between the two limiting refractive indices. This can be measured by the retardance, where

Coherent means that the waves are able to recombine with each other to give interference effects. Birefringence is retardance divided by specimen thickness, which cannot usually be measured. Retardance is a more practical measurement than birefringence and may be expressed as phase difference, by dividing linear distance by wavelength of the light. For example, a retardance of 100 nm in violet light of wavelength 400 nm is a quarter-wave phase difference.

Vectors parallel to each axis of transmission recombine after leaving a birefringent object with its axes at 45° to the plane of linearly polarized light. When the retardance is nil or equals a particular wavelength, or a multiple of it, the light of that wavelength remains linearly polarized in the plane of the polarizer and cannot pass the crossed analyzer, giving a black band in the spectrum. This is destructive interference. Except at retardances of 0 nm, this only begins to be significant at retardances approaching 400 nm, above which in white light a series of colors is seen, called Newton's scale, as various wavelengths or combinations of them are extinguished. 6 Destructive interference has been suggested as the explanation of the color of Congo red-stained amyloid between crossed polarizer and analyzer. 7

For other retardances, light generally becomes elliptically polarized. The tip of the light wave, viewed end-on, now traces an ellipse and not a straight line. The direction of rotation depends upon which vector emerges first. This makes an object appear bright against the dark background, because a component of the ellipse can pass a crossed analyzer (Figure 1). As the dimension of the ellipse in the plane of the analyzer increases, more light is transmitted and the maximum is achieved when there is half-wave retardance, or one wave and a half, and so on. Birefringent effects are maximal when the axes of transmission are at 45° to the plane of polarization between crossed polarizer and analyzer, and they decline with rotation of object or polarizer or analyzer, disappearing when the axes are parallel and perpendicular to the plane of polarization (Figure 2).

Production of elliptically polarized light by a birefringent object. When the object, such as an unstained collagen bundle, has an axis of transmission (t) at 45° between crossed polarizer (p) and analyzer (a), the vector of light waves in the slow axis is retarded more than the vector at right angles in the fast axis. The vectors recombine on leaving the object to give elliptically polarized light, some of which is in the plane of the analyzer, and can be transmitted, to make the object appear bright against a dark background.

Effect of rotation of a birefringent object between crossed polarizer (p) and analyzer (a). The object, such as an unstained collagen bundle, appears maximally bright when its axes of transmission are at 45° to the plane of polarization. The object appears as dark as the background when rotated to be parallel to the plane of the polarizer, as illustrated, or parallel to the plane of the analyzer.

Retardance may be measured with a compensator, so called because it compensates a phase difference, converting elliptically polarized light to linearly polarized. One type, Brace–Köhler, is a birefringent plate with known small retardance, inserted between the specimen stage and analyzer. This can measure the relatively slight retardances found in biological material and is rotated perpendicularly to the light beam to give polarized light of variable ellipticity. When the ellipse is opposite in direction of rotation and equal in dimensions to the ellipse produced by the birefringent object, linearly polarized light is produced in the plane of the polarizer, which is not transmitted by a crossed analyzer. This procedure is made with monochromatic light, meaning the light of a single wavelength, in case not all wavelengths are compensated at any setting of the compensator. Full compensation is when the object becomes maximally dark. This is difficult to judge because the background lightens as the analyzer transmits elliptically polarized light when the compensator is rotated. An easier point to detect is when intensity of object and background match, called half compensation.

Dispersion of the Refractive Index, and Anomalous Colors

For transparent materials, such as glass, the refractive index is nearly constant throughout the spectrum, showing a slight decline with increasing wavelength. This is normal dispersion of the refractive index, which explains the colored spectrum seen when white light passes through a triangular prism. In a birefringent material that does not absorb light, such as unstained collagen bundles, the refractive index of both axes shows normal dispersion. The birefringence is constant at every wavelength, giving white against a dark background in white light.

In contrast, any light-absorbing medium has a marked change in refractive index around an absorption peak. The refractive index falls rapidly to a minimum on the shortwave side and jumps to a maximum on the longwave side, decreasing after that, rapidly at first and then gradually as the wavelength increases. This discontinuity is called anomalous dispersion of the refractive index (Figure 3). 1, 4, 6 In practice, damping effects within the atoms and use of narrow waveband filters that cover more than one wavelength mean that the minimum and maximum values are not achieved and that observed curves are smoother than theoretical curves. 4

Anomalous dispersion of the refractive index. In a light- absorbing medium, the refractive index changes with wavelength. Away from an absorption peak, indicated by the vertical dotted lines, in which in theory strong absorption means the refractive index cannot be measured, there is a gradual decrease in refractive index with increasing wavelength. Around the peak, there are marked changes. On the shortwave side, the index falls rapidly. On the longwave side, the index is high and falls rapidly at first. In practice, the change of refractive index on each side of the peak is less marked than shown.

In an orientated, birefringent, colored material, the non-absorbing axis shows normal dispersion, while the absorbing axis shows anomalous dispersion. The birefringence is not constant at every wavelength but varies because the difference between the refractive indices of the axes changes in relation to anomalous dispersion. The transmitted color in white light depends on the net effect of birefringence at different wavelengths, and could be anomalous, meaning a color different from that seen in unpolarized white light, not explained by Newton's scale of interference colors. 6

By convention, birefringence is positive if the higher refractive index is for light polarized parallel to a feature such as long axis of a fiber or direction of smearing of a dye, and negative otherwise. A material could have positive birefringence at some wavelengths and negative at others, because in some circumstances the slow and fast axes of transmission could exchange positions. The Brace–Köhler compensator not only measures retardance but also identifies the direction of the axes of transmission.

Another mechanism that has been suggested to explain the brightness of Congo red-stained amyloid between crossed polarizer and analyzer is optical activity, or optical rotation. 8, 9 This is rotation of the plane of linearly polarized light, independently of the plane of illumination. 10 Linearly polarized light can be considered a mixture of two coherent beams of circularly polarized light, left- and right-handed. If a material has a different refractive index for the two components, there is circular birefringence, which explains optical rotation. Differential absorption of left- and right-handed circularly polarized light is circular dichroism. Optical rotatory dispersion is variation of optical rotation with wavelength. Dispersion changes markedly around an absorption peak and is called anomalous, with a change from positive to negative rotation. Combined changes in dispersion and circular dichroism around an absorption peak are the Cotton effect. Optical activity and circular dichroism typically have 1000-fold weaker effects than linear birefringence. 11

Plan of the Investigation

Optical properties were examined of sections containing amyloid stained by Congo red, and of slides that carried Congo red orientated by smearing. These properties were investigated with either a polarizer alone or an analyzer alone, or both, with rotation of the specimen and/or a polarizing filter, to determine dichroic and birefringent effects (Figure 4). The absorption spectrum of Congo red was measured in solution, and in the dye orientated by smearing, in various conditions of polarized light. Birefringence of sections and smears was quantified by measurement of retardance at a range of wavelengths through the visible spectrum. The hypothesis was that variation of birefringence around the absorption peak of Congo red explained the observed properties.

Arrangement of components on a microscope for polarizing microscopy. The only component not in everyday use is a Brace–Köhler compensator. Some microscopes have a rotatable analyzer and fixed polarizer, rather than vice versa. Depending on the investigation, any or all of polarizer, compensator, and analyzer may be removed from the light path.

Other suggested mechanisms were investigated. Calculation was used to assess the contribution of rotation of the plane of linearly polarized light by unequal absorption in two axes. 5 Measurement of retardance showed whether this was large enough to give destructive interference. 7 Optical activity and the Cotton effect would allow an analyzer to transmit light whatever was the plane of polarized light, and if these mechanisms were significant, smears or stained amyloid fibrils should never appear as dark as the background between crossed polarizer and analyzer. 8, 9

Color Contrast

In this investigation, you'll discover how colors seem to change when you place them against different-colored backgrounds. You need to consider this phenomenon when you pick out colors for carpeting or walls or when you're painting a picture.

Tools and Materials

  • Scissors
  • Construction or origami paper in yellow, purple, green, blue (two shades), and orange (two shades) select pieces of paper that are the same size
  • Glue
  • Paint-sample cards (from paint or hardware stores) that show gradations of one color


  1. Cut one sheet of orange paper in half lengthwise and glue it to cover up half of a blue sheet. This will give you a large sheet of paper that’s half blue and half orange. This large piece of paper will be your background for other colors.
  2. Cut two small squares from each of the colors you have, including squares of blue or orange of a different shade from that of the large sheets.
  3. Make two matching, evenly spaced columns of colored squares, one on the blue background and one on the orange background (see image above). Glue each of the corresponding squares in place on the blue and orange backgrounds.
  4. From the same colors as the small squares, cut a strip of each color wide enough that it can be placed over both columns at once for comparison. The illustration below shows two identical sample squares on two different backgrounds, and a comparison bar (click to enlarge).

To Do and Notice

Notice that two small squares of the same color may appear to be different shades when mounted on different-colored backgrounds. Place the comparison strip so that it touches both small squares of color at the same time to verify that the squares are actually the same color. Experiment with different colors to see which background colors make foreground colors appear lighter and which make them appear darker.

Color contrast also works in reverse: Against certain backgrounds, different colors can look the same. From the paint samples, choose two shades that are very similar but are clearly distinguishable when placed right next to each other. Put the paint samples on different backgrounds. The slightly different colors may then appear to be the same. You will have to experiment with different backgrounds to get the desired effect.

What’s Going On?

The back of your eye is lined with light-sensitive cells, including color-sensitive cone cells. Your cones affect each other in complex ways. These connections give you good color vision, but they can also fool your eye.

When cones in one part of your eye see blue light, they make nearby cones less sensitive to blue. Because of this, you see a colored spot on a blue background as less blue than it really is. If you put a purple spot on a blue background, for instance, the spot looks a little less blue than it otherwise would. Similarly, a red spot on an orange background looks less orange than it otherwise would.

Going Further

When nineteenth-century astronomers observed Mars through telescopes, they saw a wave of green spread down from the planet’s north pole as the polar cap disappeared each spring. Modern astronomers know that this wave of green is actually gray volcanic dust spread by carbon dioxide expanding from the dry ice of the polar cap. A red background makes gray spots look greenish. The gray dust of Mars appeared green to human eyes when it was viewed against the planet’s red background.

Gram Staining

The Gram stain procedure is a differential staining procedure that involves multiple steps. It was developed by Danish microbiologist Hans Christian Gram in 1884 as an effective method to distinguish between bacteria with different types of cell walls, and even today it remains one of the most frequently used staining techniques. The steps of the Gram stain procedure are listed below and illustrated in Figure (PageIndex<3>).

  1. First, crystal violet, a primary stain, is applied to a heat-fixed smear, giving all of the cells a purple color.
  2. Next, Gram&rsquos iodine, a mordant, is added. A mordant is a substance used to set or stabilize stains or dyes in this case, Gram&rsquos iodine acts like a trapping agent that complexes with the crystal violet, making the crystal violet&ndashiodine complex clump and stay contained in thick layers of peptidoglycan in the cell walls.
  3. Next, a decolorizing agent is added, usually ethanol or an acetone/ethanol solution. Cells that have thick peptidoglycan layers in their cell walls are much less affected by the decolorizing agent they generally retain the crystal violet dye and remain purple. However, the decolorizing agent more easily washes the dye out of cells with thinner peptidoglycan layers, making them again colorless.
  4. Finally, a secondary counterstain, usually safranin, is added. This stains the decolorized cells pink and is less noticeable in the cells that still contain the crystal violet dye.

Figure (PageIndex<1>): Gram-staining is a differential staining technique that uses a primary stain and a secondary counterstain to distinguish between gram-positive and gram-negative bacteria.

The purple, crystal-violet stained cells are referred to as gram-positive cells, while the red, safranin-dyed cells are gram-negative (Figure (PageIndex<4>)). However, there are several important considerations in interpreting the results of a Gram stain. First, older bacterial cells may have damage to their cell walls that causes them to appear gram-negative even if the species is gram-positive. Thus, it is best to use fresh bacterial cultures for Gram staining. Second, errors such as leaving on decolorizer too long can affect the results. In some cases, most cells will appear gram-positive while a few appear gram-negative (as in Figure (PageIndex<4>)). This suggests damage to the individual cells or that decolorizer was left on for too long the cells should still be classified as gram-positive if they are all the same species rather than a mixed culture.

Besides their differing interactions with dyes and decolorizing agents, the chemical differences between gram-positive and gram-negative cells have other implications with clinical relevance. For example, Gram staining can help clinicians classify bacterial pathogens in a sample into categories associated with specific properties. Gram-negative bacteria tend to be more resistant to certain antibiotics than gram-positive bacteria. We will discuss this and other applications of Gram staining in more detail in later chapters.

Figure (PageIndex<4>): In this specimen, the gram-positive bacterium Staphylococcus aureus retains crystal violet dye even after the decolorizing agent is added. Gram-negative Escherichia coli, the most common Gram stain quality-control bacterium, is decolorized, and is only visible after the addition of the pink counterstain safranin. (credit: modification of work by Nina Parker)

  1. Explain the role of Gram&rsquos iodine in the Gram stain procedure.
  2. Explain the role of alcohol in the Gram stain procedure.
  3. What color are gram-positive and gram-negative cells, respectively, after the Gram stain procedure?


Viewing Cindy&rsquos specimen under the darkfield microscope has provided the technician with some important clues about the identity of the microbe causing her infection. However, more information is needed to make a conclusive diagnosis. The technician decides to make a Gram stain of the specimen. This technique is commonly used as an early step in identifying pathogenic bacteria. After completing the Gram stain procedure, the technician views the slide under the brightfield microscope and sees purple, grape-like clusters of spherical cells (Figure (PageIndex<5>)).

  1. Are these bacteria gram-positive or gram-negative?
  2. What does this reveal about their cell walls?

Figure (PageIndex<5>): Cindy's specimen (credit: modification of work by American Society for Microbiology)

8 Answers 8

Most computer monitors aren't capable of displaying any spectral color. Some of the RGB monitors could display at most three of them: some red wavelength, some green and some blue. This is because the gamut of the human vision is not triangular, instead it's curved and resembles a horseshoe:

In the image above, the black curve represents the spectral colors, with the wavelengths in nm denoted by green numbers. The colored triangle is the sRGB gamut, the standard gamut that most "usual" computer monitors are supposed to have.

As you can see, the black curve doesn't even touch the triangle, which means that sRGB monitors can't display any of the corresponding colors.

This doesn't mean that you can't see any good representation of the visible spectrum. You can e.g. display what the spectrum would look like if you took a gray card and projected the spectrum onto it, thus getting a desaturated version. CIE 1931 color space, via its color matching functions, lets one find, for each spectral color, corresponding color coordinates $XYZ$ , which then can be converted to the coordinates in other color spaces like the above mentioned sRGB.

The inability of the sRGB monitors to display spectral colors manifests in the fact that, after you convert $XYZ$ coordinates to sRGB's $RGB$ ones, you'll get some negative components. Of course, negative amount of light is not something a display device can emit, so it needs some workaround to display these colors (or something close to them). Displaying the spectrum as projected on a gray card is one of these workarounds.

Here's how such a desaturated spectrum (with a scale) would look:

To get this (or any other, actually) image to display "correctly", ideally you need to calibrate your monitor. Some of the consumer devices have better color rendering out of the box, others have quite poor color rendering and show visibly wrong colors. If you don't calibrate, then just be aware of this nuance.

Also, if you happen to be a tetrachromat (virtually never happens in males, rare in females), then the above image will look incorrect to you in any case.

How to see an actual spectrum, without the workarounds discussed above? For this you should use not a computer monitor. Instead you need a spectroscope. These can be found in online stores like AliExpress quite cheap, some using a diffraction grating, others a prism. The ones with grating will give you almost linear expansion in wavelengths, while the ones with a prism will have wider blue-violet part and thinner orange-red part of the spectrum.

Why does white light viewed through optical chopper appear purple/blue? - Biology

The colors in an image will depend on what kind of light the satellite instrument measured. True-color images use visible light&mdashred, green and blue wavelengths&mdashso the colors are similar to what a person would see from space. False-color images incorporate infrared light and may take on unexpected colors. In a true color image, common features appear as follows:


Water absorbs light, so it is usually black or dark blue. Sediment reflects light and colors the water. When suspended sand or mud is dense, the water looks brown. As the sediment disperses, the water&rsquos color changes to green and then blue. Shallow waters with sandy bottoms can lead to a similar effect.

Sunlight reflecting off the surface of the water makes the water look gray, silver, or white. This phenomenon, known as sunglint, can highlight wave features or oil slicks, but it also masks the presence of sediment or phytoplankton.

Sunglint makes it possible to see current patterns on the ocean&rsquos surface around the Canary Islands. (NASA image courtesy Jeff Schmaltz LANCE/EOSDIS MODIS Rapid Response Team, GSFC.)

Frozen water&mdashsnow and ice&mdashis white, gray, and sometimes slightly blue. Dirt or glacial debris can give snow and ice a tan color.


Plants come in different shades of green, and those differences show up in the true-color view from space. Grasslands tend to be pale green, while forests are very dark green. Land used for agriculture is often much brighter in tone than natural vegetation.

In some locations (high and mid latitudes), plant color depends on the season. Spring vegetation tends to be paler than dense summer vegetation. Fall vegetation can be red, orange, yellow, and tan leafless and withered winter vegetation is brown. For these reasons, it is helpful to know when the image was collected.

The forests covering the Great Smoky Mountains of the Southeastern United States change colors from brown to green to orange to brown as the seasons progress. (NASA images courtesy Jeff Schmaltz LANCE/EOSDIS MODIS Rapid Response Team, GSFC.)

In the oceans, floating plants&mdashphytoplankton&mdashcan color the water in a wide variety of blues and greens. Submerged vegetation like kelp forests can provided a shadowy black or brown hue to coastal water.

Bare ground

Bare or very lightly vegetated ground is usually some shade of brown or tan. The color depends on the mineral content of the soil. In some deserts such as the Australian Outback and the southwestern United States, exposed earth is red or pink because it contains iron oxides like hematite (Greek for blood-like). When the ground is white or very pale tan, especially in dried lakebeds, it is because of salt-, silicon-, or calcium-based minerals. Volcanic debris is brown, gray, or black. Newly burned land is also dark brown or black, but the burn scar fades to brown before disappearing over time.


Densely built areas are typically silver or gray from the concentration of concrete and other building materials. Some cities have a more brown or red tone depending on the materials used for rooftops.

The contrast between Warsaw&rsquos modern and historic neighborhoods is easily visible by satellite. The new Stadion Narodowy is brilliant white. &Sacuteródmie&sacutecie (Inner City) was rebuilt after World War II and most areas appear beige or gray. But some neighborhoods rebuilt with older-style buildings, such as the red tile and green copper roofs of Stare Miasto (Old Town). (Image courtesy NASA/USGS Landsat.)


Clouds are white and gray, and they tend to have texture just as they do when viewed from the ground. They also cast dark shadows on the ground that mirror the shape of the cloud. Some high, thin clouds are detectable only by the shadow they cast.

Smoke is often smoother than clouds and ranges in color from brown to gray. Smoke from oil fires is black. Haze is usually featureless and pale gray or a dingy white. Dense haze is opaque, but you can see through thinner haze. The color of smoke or haze usually reflects the amount of moisture and chemical pollutants, but it&rsquos not always possible to tell the difference between haze and fog in a visual interpretation of a satellite image. White haze may be natural fog, but it may also be pollution.

Clouds, fog, haze and snow are sometimes difficult to distinguish in satellite imagery, as in this MODIS image of the Himalaya from November 1, 2013. (Image adapted from MODIS Worldview.)

Dust ranges in color, depending on its source. It is most often slightly tan, but like soil, can be white, red, dark brown, and even black due to different mineral content.

Volcanic plumes also vary in appearance, depending on the type of eruption. Plumes of steam and gas are white. Ash plumes are brown. Resuspended volcanic ash is also brown.

Colors in Context

Looking at a satellite image, you see everything between the satellite and the ground (clouds, dust, haze, land) in a single, flat plane. This means that a white patch might be a cloud, but it could also be snow or a salt flat or sunglint. The combination of context, shape, and texture will help you tell the difference.

For example, shadows cast by clouds or mountains can be easy to mistake for other dark surface features like water, forest, or burned land. Looking at other images of the same area taken at another time can help eliminate confusion. Most of the time, context will help you see the source of the shadow&mdasha cloud or mountain&mdashby comparing the shape of the shadow to other features in the image.

Find North

When you get lost, the simplest way to figure out where you are is to find a familiar landmark and orient yourself with respect to it. The same technique applies to satellite images. If you know where north is, you can figure out if that mountain range is running north to south or east to west, or if a city is on the east side of the river or the west. These details can help you match the features to a map. On the Earth Observatory, most images are oriented so that north is up. All images include a north arrow.

Consider your Prior Knowledge

Perhaps the most powerful tool for interpreting a satellite image is knowledge of the place. If you know that a wildfire burned through a forest last year, it&rsquos easy to figure out that the dark brown patch of forest is probably a burn scar, not a volcanic flow or shadow.

Land burned by Yosemite&rsquos Rim Fire is gray brown in comparison to the unburned brown and green landscape around it. See this linked map that helps differentiate between burned land and non-burned land. (NASA Earth Observatory images by Robert Simmon, using Landsat 8 data from the USGS Earth Explorer.)

Having local knowledge also allows you to connect satellite mapping to what&rsquos happening in everyday life, from social studies, economics, and history (for example, population growth, transport, food production) to geology (volcanic activity, tectonics) to biology and ecology (plant growth and ecosystems) to politics and culture (land and water use) to chemistry (atmospheric pollution) and to health (pollution, habitat for disease carriers).

For example, land ownership and land use policy is contrasted in the pair of images below. In Poland, small parcels of privately owned land surround the Niepolomice Forest. The government has managed the forest as a unit since the thirteenth century. While the canopy isn't a solid, unbroken green, the forest is largely intact. The lower image shows a checkerboard combination of private and public land near Washington&rsquos Okanogan-Wenatchee National Forest. The U.S. Forest Service manages the forest under a mixed use policy that preserves some forest, while opening other sections to logging. Lighter green areas indicate that logging has occurred on federal, state, or private land. Parcels of private land are much larger in this part of the western United States than in Poland.

Land use and conservation policies define the forest area in both Poland (top) and the U.S. state of Washington (lower). (NASA Earth Observatory images by Robert Simmon, using Landsat 8 data from the USGS Earth Explorer.)

If you lack knowledge of the area shown, a reference map or atlas can be extremely valuable. A map gives names to the features you can see in the image, and that gives you the ability to look for additional information. Several online mapping services even provide a satellite view with features labeled. Historic maps, such as those found at the Library of Congress or in the David Rumsey Map Collection, can help you identify changes and may even help you understand why those changes occurred.

Whether you are looking at Earth for science, history, or something else, also consider the Earth Observatory as a key resource. The site hosts a rich, deep archive of more than 12,000 interpreted satellite images covering a wide range of topics and locations. The archive includes images of natural events as well as more diverse featured images. If the Earth Observatory does not have an image of an area or topic that interests you, please let us know. We&rsquore always looking for new ways to explore our world from space.

The Ponzo Illusion

In the Ponzo illusion, two identically-sized lines appear to be different sizes when placed over parallel lines that seem to converge as they recede into the distance.  

What Do You See?

In the image above illustrating the Ponzo illusion, the two yellow lines are the exact same size. Because they are placed over parallel lines that seem to converge in the distance, the top yellow line actually appears to be longer than the bottom one.

How Does the Ponzo Illusion Work?

The Ponzo illusion was first demonstrated in 1913 by an Italian psychologist named Mario Ponzo. The reason the top horizontal line looks longer is that we interpret the scene using a linear perspective. Since the vertical parallel lines seem to grow closer as they move further away, we interpret the top line as being further off in the distance. An object in the distance would need to be longer in order for it to appear the same size as a near object, so the top "far" line is seen as being longer than the bottom "near" line, even though they are the same size.

When do we see which colour?

Figure 25
Idealized schematic remission curves of a blue, green and red surface. If only the short wavelengths are strongly remitted (blue line), the S-receptors are stimulated strongly and the M-receptors only weakly, and the resulting sensation is "blue". The primary sensations "green" and "red" arise when the stimulation of the M- or L-cones, respectively, strongly exceeds that of the other ones.

Figure 26
Idealized transmission curves of layers of inks as used for colour printing (or remission curves of white paper with yellow, magenta or cyan ink on it). The yellow ink absorbs strongly the short waves and lets middle and long waves pass, magenta absorbs most strongly in the middle range and cyan in the long-wavelength region. Yellow is, so to say, white minus blue, magenta is white minus green, and cyan is white minus red.

Blood Smear in Microscopy

In general, performing and viewing a b lood smear for microscopy analysis is usually warranted when the hematology analyz ers employed in laboratories indicate some abnormalities.

Here at MicroscopeMaster , the goal is to not perform diagnoses but to briefly outline the technique and processes needed to view a blood smear under brightfield microscopy for the hobbyist who enjoys furthering their knowledge or for the student needing some clarification.

Hematology analyzers provide complete blood cell counts and differential counts of white blood cells estimating cell number but offer limited morphological information and do not identify abnormalities.

Blood Smear Process and Technique

Importantly, v iewing blood smears under the microscope needs to be done shortly after blood collection employing sterile technique (**wearing gloves) from a disinfected site (wiping off 1st drop of blood). Using a high quality clean glass slide (flat, no distortions and corrosion resistant) of 75mm X 25mm and 1mm thickness is ideal . T hen plac e the blood drop 1cm from the end of the slide.

P roceeding with the 45 degree wedge or push slide technique used in manual and automated environments, creates a monolayer blood smear. This is done i n a smooth and quick motion. Fixation, staining, washing and air drying are quickly commenced.

Below is a very quick video on blood smear technique:

When staining either R omanowsky , W right ' s or G iemsa stain are used or a combination thereof, u sually simply Wright's stain.

Using light microscopy, imaging is done with a 10x ocular and 10x objective at first.

Increasing in p rogression, it is best to view smears up to a total magnification of 1000x using the 10x ocular.

Red blood cells will stain pink , platelets appear as small blue/purple and cytoplasmic g ranules stain pink to v iolet. W hite blood cells comprise Granulocytes which include Neutrophils, Eosinophils and Basophils and Agranulocytes which include Lymphocytes and Monocytes . N eutrophils are displayed as a deep blue/purple nucleus under the microscope.

Artifacts / Refractiles

If when viewing your dried blood smear through a microscope you are seeing refractiles , there are some questions to ask yourself and factors to consider. Firstly, what stain and technique are you using? Perhaps there is a problem with the optics and magnifications. What are the se specification s ? Lastly, p erhaps your problem is with the specimen or your specimen preparation like poor spreading technique.

You may be seeing a drying related artifact or a stain related artifact which are usually more evident during humid summer months. Internal reflections or contamination in the sample and/or in the optics can cause refractiles . M ismatched components/mechanisms on your microscope may also be a contributing factor .

If doing a Complete Blood Count with differential, artifacts may be ignored as they should not affect your counts including red cell morphologies . An experienced observer should be comfortable in doing this so as to deliver results in a lab accurately and quickly.

Causes of refractiles : Humidity under-fixation/late fixation presence of water in alcohol used excess buffer to stain, thick smear

Elimination of refractiles : Av oidance of any humidity thin smear dehydraton set-up

After being correctly smeared and dried, water is lost from thin areas first so as from the plasma, then water is lost from within the cells. Thicker areas dry from outside to inside. W ith an inactivated cell membrane, loose water can no longer disperse from the cell unless in the case of rupture which will then easily destroy cell morpholog ies . A cause of refractiles due to inadequate drying can be trapped water in an inactive cell membrane.

Refractiles may be avoided with more rapid heat ing and then drying before staining. Perhaps simply waving the slide around immediately after placing the smear to speed drying will prove sufficient .

An artifact caused by staining is more difficult to remedy.

Live Blood Analysis Refractiles

In live blood analysis , much the same questions and factors can be considered. A new piece of optical equipment /dye may alter your signal-image. For example, the autofluorescense in hemoglobin may disrupt your emission signal from your fl uo rescent dye.

If halos are present causing refractiles because of autofluorescence then you should consider the addition of either trypan blue, methylene blue or Toluidine blue to your stain.

A blood smear is easily sampled and features of blood are very observable which is especially valuable when disease is present . It is vital to keep in mind sterility, quality control and proper technique in handling your blood smear.

Optical microscopy offers the ability to gather information and aid in continued research . Thus, MicroscopeMaster hopes to have shed some light.

Visible spectrum

The visible spectrum is a very small portion of the electromagnetic spectrum that can be seen by the human eye. This is summarized in the figure below, which is not to scale. The exact boundaries of each type of wave are not precisely defined and often different authors have slightly different values. This is not really relevant since this classification is just indicative.

The electromagnetic spectrum (not to scale).

As frequency increases the wavelength becomes smaller and the energy associated to each photon increases. Visible light, infrared and radiation with longer wavelengths are non-ionizing radiations. UV light, X rays, gamma rays and radiations with shorter wavelength are ionizing radiations, meaning that each photon carries enough energy to ionize matter. When a ionizing photon with enough energy impinges on an atom or on a molecule, it can knock off one electron from it. Having lost an electron, the atom (or molecule) becomes electrically charged and is called an ion. Because of their electrical charge, ions are chemically very active and tend to react with nearby molecules. The limit between ionizing and non-ionizing radiations is located somewhere in the UV region, but it's not clearly defined and somehow fuzzy. A photon energy 1 of 10 eV (corresponding to a wavelength of 124 nm) is often considered the threshold between ionizing and non-ionizing radiation, but there is no consensus on this topic.

The wavelengths 2 of the visible spectrum are usually between 400 nm and 700 nm. The energy carried by each visible photon is between 3.1 eV and 1.8 eV respectively and the frequency is in the 750 THz to 428 THz range

The sensitivity of the eye to wavelengths beyond this range drops dramatically and they are represented as black in the figure below. The maximum sensitivity is usually between 500 and 550 nm.

The perceived color dependens on the wavelength and the table below will give a rough idea. Of course, this varies from one person to another.

Red 700. 630 nm
Orange 630. 600 nm
Yellow 600. 570 nm
Green 570. 530 nm
Cyan 530. 490 nm
Indigo 490. 450 nm
Blue 450. 400 nm

You may be surprised that the color called violet actually looks blue and that there is no purple nor pink: this is because in the 17 th century when Newton discovered the spectrum of the sunlight, the names of the colors corresponded to slightly different hues. He called violet what we would call blue today and blue what we would call cyan. You may also think at the famous poem "roses are red and violets are blue", the hue of the violets is actually blue.

Purple and pink, on the other hand, are colors that do not correspond to an unique wavelength, they are perceived by eye when both blue and red light are superposed. Since blue and red are at opposite sides of the visible spectrum, no single wavelength will appear purple or pink.

Talking about missing colors, there is no brown nor grey light neither: these colors are just perceived when compared with other brighter colors, gray is a dim white and brown is a dim yellow/orange.

Wavelengths shorter than 390 nm are part of the UV spectrum and are not visible UV meaning ultra violet, beyond violet (meaning blue). Wavelengths longer than 750 nm are part of the IR spectrum and are not visible neither IR meaning infrared, below red. It's not possible to give a precise boundary between visible and invisible wavelengths since that's very subjective: some people may still see some light at 720 nm and some others may not.

White light, a superposition of all colors, can be decomposed with a prism to observe its spectrum. Just shining light at a prism is not enough the light has to go through a narrow slit to let only a small ray pass through. The quality of this slit is very important for the precision of the result. In the example below, the slit is just a crude cut in a piece of cardboard and makes a very rough spectrometer, but still enough to observe a "rainbow".

Visible spectrum of white light observed with a prism.

Watch the video: Understanding Absorption of Light - Why do we see different colors? (January 2022).