The first article examined the relationship between wavelength and the color of light. It is now time to examine how light interacts with objects to produce color and how that interaction is dependent on wavelength. There are several ways in which this interaction can occur. This article will cover only three: reflection, transmission, and interference.
When most people think of light illuminating an object and revealing its color, they are actually thinking of the process of reflection. It seems like a fairly simple process. Someone turns on a light in a room. The light travels from a lamp to an apple on a table. The light reflects off the apple. Anyone that enters the room sees the reflected light and determines that the apple is red. However, it is actually a bit more complicated than that. In reality, the process of reflection is tied to a process called absorption.
Figure 1 illustrates the process of reflection. In this case, an object is illuminated by light. The light contains different wavelengths as represented by the red, green, and blue waves. When the light hits the object, one of two things happens to the light. Some of the wavelengths are absorbed by the object. This is shown in the figure by the red and green waves. These waves penetrate the object for a certain distance and are absorbed by the object. The wavelengths that are not absorbed are reflected. This is shown in the figure by the blue wave. The reflected wavelengths are what we see. Thus, a blue object is blue because it reflects the blue wavelengths. However, at a deeper level, it is actually the absorption of light that gives an object its color. A blue object is blue because the object absorbed the other wavelengths of the light leaving only the blue light to reflect.
The important thing here is that the absorption of light depends on its wavelength. Some wavelengths are absorbed and some are reflected. One might reasonably wonder what causes some wavelengths to be absorbed by an object and others to be reflected. The answer is found in the chemical composition of the object. The interaction of the light and the object occurs at the molecular level. Thus, the molecular structure of the object determines which wavelengths are absorbed. One might also wonder what happens to the wavelengths that are absorbed. A number of different things can happen to the absorbed light. However, one of the most common is that the light is turned into heat. This is demonstrated every hot, summer day. We all know that on summer days black objects become very hot. For instance, black cars get much hotter than white cars. This is because of the absorption of light. The reason that white cars are white is because they reflect all the colors of light. In other words, all of the wavelengths of light are reflected by white cars and very little is absorbed. Since little light is absorbed, little of the light gets turned into heat. On the other hand, black cars are black because they absorb most of the colors of light. Since most of the light is absorbed, most of the light gets turned into heat and the car becomes very hot.
When working with the interaction of light and objects, it is often important to be able to show the wavelengths (colors) that an object reflects. This can be done with a spectral reflectance curve. A spectral reflectance curve shows the amount of each wavelength of light that an object reflects.
Figure 2 shows a spectral reflectance curve. The wavelength is indicated along the horizontal axis and the reflectance (relative amount of light reflected) is shown along the vertical axis. As can be seen, the object represented by this curve reflects primarily blue light. Spectral reflectance curves are very similar to the spectral curves that were introduced in the previous article. The difference is that the spectral curve shows the wavelengths that a light contains while a spectral reflectance curve shows the wavelengths that are reflected by an object.
It would be tempting to say that the light reflected by an object is determined by the object's spectral reflectance curve (e.g., the object represented by Figure 2 would reflect primarily blue light). However, the spectral reflectance curve is only half the story.
To completely understand the color of light reflected from an object, we have to consider both the spectral reflectance curve of the object and the spectral curve of the light illuminating the object. This is demonstrated in Figures 3 -- 5.
The key to understanding Figure 5 is to realize that an object can only reflect a wavelength if the light that illuminates the object contains that wavelength. In this figure, the spectral reflectance curve of the object shows that the object reflects mostly green and yellow. However, the spectral curve of the light shows that the light does not contain any green or yellow. Thus, the object can not reflect any of these wavelengths. The object can only reflect wavelengths where the two curves overlap. Thus, this object will reflect mostly cyan light. Since the area of overlap between the two curves is so small, this object will reflect only a small amount of light and will appear very dim.
So, the moral of the story is that the light reflected from an object is not determined solely by the reflectance properties of the object but by both the reflectance properties of the object and the spectral curve of the light illuminating the object.
Another way that color manifests is through the process of transmission. With transmission, light is actually transmitted through an object. One example of transmitted light is colored glass. Another example is backlit, semitransparent objects such as a flower. As in the case of reflection, transmission is also tied to the process of absorption.
Figure 6 illustrates the process of transmission. In this figure, a transparent object is illuminated by light. The light contains different wavelengths represented by the red, green, and blue waves. When the light hits the object, one of two things happens to the light. Some of the wavelengths are absorbed by the object. This is shown in the figure by the green and blue waves. These waves penetrate the object for a certain distance and are absorbed by the object. The wavelengths that are not absorbed are transmitted. This is shown in the figure by the red wave. The transmitted wavelengths are what we see. As in the case of reflected light, the wavelengths that we see are really determined by the absorption of light that occurs in an object. Thus, an object that transmits red light is red because the object absorbed the other wavelengths of the light leaving only the red wavelength to transmit.
As previously, the absorption of light depends on its wavelength. Some wavelengths are absorbed and some are transmitted (which is determined by the object's chemical composition at a molecular level).
It is often important to be able to show the wavelengths (colors) that an object transmits. This can be done with a spectral transmission curve. A spectral transmission curve shows the amount of each wavelength of light that an object transmits.
Figure 7 shows a spectral transmission curve. The wavelength is indicated along the horizontal axis and the transmission (relative amount of light transmitted) is shown along the vertical axis. This spectral transmission curve shows that the object represented by the curve transmits blue, cyan, green, yellow, and some red.
Above, we saw that the light that is reflected from an object is a function of both the reflective properties of an object and the light that illuminates the object. A similar situation exists with transmission. The light that is transmitted from an object is a function of both the transmission properties of the object and the light that illuminates the object. To see the interaction of a translucent object and light, the spectral transmission curve of the object and the spectral curve of the light illuminating the object must be examined. This is demonstrated in Figures 8 -- 10.
Interference is an interesting phenomenon that can occur with reflected light under a certain set of conditions. There are actually two types of interference: constructive interference and destructive interference. These two types of interference occur at the same time. You are already familiar with interference. For instance, the colors that are seen when an oily film forms on a pool of water are an example of color produced by interference.
Figure 11 illustrates the process of interference. For interference to occur, there must be an object (such as a body of water) with a thin, transparent layer (such as a film of oil) on top. There are now two surfaces that can reflect waves of light: the surface of the top layer and the surface between the layer and the object. When a wave of light, represented by the blue ray in the figure, hits the top surface of the layer, two things happen. Part of the wave is reflected. The part that is not reflected is transmitted through the wave to the surface between the layer and the object. At least part of this wave is then reflected at this interface (some of it can be transmitted or absorbed). As a result, there are now two reflected waves.
To appreciate what happens next, it is necessary to understand a little bit about wave theory. Each wave has what is called a phase. In simplistic terms, the phase of a wave refers to the position of the wave.
It is easy to compare the phases of waves. Figure 12 shows two waves that are perfectly in phase (to simplify the explanation, these waves have the same wavelength). When the waves are in phase, the crests and troughs of the waves align with each other. When this happens, the waves reinforce each other to produce a new wave which has higher crests and lower troughs. In other words, the waves add together to produce a new wave that is twice as tall as the original waves. Thus, the new wave is bigger, and the color from the wave is brighter. This is called constructive interference -- the waves constructively interfere with each other to produce a new, stronger wave.
Figure 13 shows two waves that are perfectly out of phase (again, to simplify the explanation, these waves have the same wavelength). When the waves are out of phase, the crests of one wave are aligned with the troughs of the other. When this happens, the two waves cancel each other. This destroys the two waves. Thus, there will be no light from the waves. This is called destructive interference -- the waves destroy each other so that there is no light.
Figures 12 and 13 show waves that are perfectly in phase and perfectly out of phase. In the real word, waves can be in-between these two situations so that the waves either partly reinforce or partly destroy each other.
Back to Figure 11. If the extra distance traveled by the wave that reflects off the second surface is exactly equal to a multiple of the wavelength of the light (i.e., the extra distance is equal to one wavelength, or two wavelengths, or three wavelengths, or so on), the two reflected waves will be perfectly in phase and the color of the wavelength of the light will become brighter due to constructive interference. On the other hand, if the extra distance traveled by the wave that reflects off the second surface is exactly equal to half a wavelength of the light (or one and a half wavelengths, or two and a half wavelengths, and so on), the two reflected waves will be perfectly out of phase and the color of the wavelength of the light will disappear. Wavelengths that fall in between these two cases will be either partly reinforced or partly destroyed depending on the relationship of the wavelength to the thickness of the layer.
Thus, with interference, some colors become brighter and some become less bright or are completely destroyed. The result is often bands of color (such as those seen with water that has an oily film on top).
One thing that should be noted is that this phenomenon only occurs with very thin films that are approximately as thick as a wavelength of light.
There are other processes in which light interacts with objects to produce color (e.g., diffraction and fluorescence). However, reflection and transmission account for most of the color that we see. Interference was also covered in this article to show how a somewhat more complicated process can actually affect what we see in real life. The other processes will not be covered as they are beyond the scope of this article.