Light and the Eye
Light from an incandescent light bulb contains all the colors of the rainbow and more. It also contains some invisible "colors" including ultraviolet (uv) and infrared (ir). Some other animals are able to see these. For example, most nocturnal animals can see some ir light that we cannot and bees are able to see uv light. What is it that distinguishes different colors of light?
This is a very broad question that has at least two different answers: one is based on our perception of colors by the eye, while a second is based on the physical properties of light. The former is clearly limited since our eyes cannot see much of the light emitted by a light bulb, but it is worth spending some time discussion how our eyes perceive light and color.
The Human Eye
Eyes are our visual window to the world. They are complex structures that are capable of very high resolution (can see small objects clearly), are extremely sensitive to light- capable even of detecting single photons (single packets, or units, of light), can sense color and depth perception and adjust to focus on objects from as close as a few inches, in young eyes, to "infinity".
Look at this web site for some nice (and needed) pictures for the following discussion: anatomy of the eye - there are also some movies to see.
Starting from the outside, the external covering of the eye consists of three layers. Most of the outermost layer is the sclera, the white of the eye, a tough fibrous layer containing nerve endings but no blood vessels. The sclera covers about 85% of the eyeball, roughly a 2.5 cm, or one inch, sphere, but the front portion consists of a transparent 1.2 cm diameter cornea. The next two inner layers are the choroid, filled with pigments and blood vessels, and the retina, the site of photon detection. Neither of these layers extend into the cornea region (see the figure on the above web site).
At the front end of the eye behind the cornea is a liquid-filled chamber with the aqueous humor, which is continually drained and replaced, bounded also by the lens and the iris. A build-up of pressure in this region can produce a condition known as glaucoma, which can lead to blindness. The lens is made from a crystalline array of 25% protein and 10% lipids and is one of the few parts of our bodies that is preserved without any turnover of cells. With age, or disease, the lens loses its perfect crystallinity and develops defects that scatter light. Known as cataracts, when sufficiently large they can adversely affect vision by ‘clouding’ the eye, just as if you tried to view the world through a thin layer of milk. The shape of the lens is controlled by ciliary muscles that can change the focusing ability of the lens in a process known as accommodation. Normally, without any shape change of the lens, we can focus on objects from about 20 feet away to infinity. To see objects closer than that distance, the eye cannot remain relaxed, but the lens shape must change to give a tighter focus.
The iris serves as an adjustable aperture, or viewing hole, and is pigmented, giving the eye its characteristic coloring. The central opening, known as the pupil, is the photon entrance path. Filling the eyeball is a gel-like material, the vitreous humor, which is more or less permanent. Six pairs of muscles control the movement of the eyeball in its socket, allowing us to focus images of interest on the highest acuity region of the retina, the fovea. This region of the retina, also known as the macula, has only cone cells, the photon receptor responsible for color vision, with each cone having its own direct connection to an individual nerve cell in the optic nerve bundle. The macula therefore has the highest spatial resolution on the retina; outside the fovea there are roughly 10 cones per nerve connection or 125 rods, the other type of photon receptor, per nerve. These nerve cells collect in the optic disc, creating a blind spot with no visual pigment, and lead to the optic nerve.The structure of the retina is shown in cross-section at retina. Note the striking fact that incident light must travel through the network of nerve cells before reaching the photoreceptors, lying partly immersed in a layer of pigmented cells. Fortunately these cells are transparent, but only about 50% of the light that strikes the cornea gets to the retina, and only about 20% of that gets to the light detecting cells. These cells, the rods and cones, permanent and not replaced over time, are, however, 100% efficient, meaning that each detected photon produces an electrical impulse along a nerve cell. Light that is not absorbed by the photoreceptors is subsequently absorbed by a layer of pigmented cells to prevent stray reflections of light within the retina. There are about 125 million rods and 7 million cones on the retina distributed such that only cones are at the central fovea, where vision is most acute. The rods and cones are named by their shapes, but are somewhat similar in overall structure. Note that the retina consumes the greatest amount of oxygen per unit weight of any tissue in the body.
Inner and outer segment portions of the rods and cones are shown in figures 8 and 9 at photoreceptors of the eye. Light transduction (changing of the light energy to electrical energy) takes place in the outer segments, the rod outer segments having been studied much more thoroughly than those of the cone. They are each 20 mm long and 2 mm in diameter (where 1 mm = 10-6 m or 0.001 mm - note: review powers of ten here) and contain stacks of rod discs that are membranes containing the visual pigment, rhodopsin, with about 100,000 rhodopsin molecules per mm2 of surface area - about the head of a pin.
Rhodopsin consists of two parts: a protein portion known as opsin, and a smaller derivative of vitamin A known as retinal, the light absorbing portion (Figure 9 at the above site). There are two possible forms of retinal: the shape found in the dark and the shape that nearly instantaneously forms after the absorption of a single photon of light. With the advent of extremely short laser pulses of light, this first step in the vision process, the structural change of retinal from the dark to the light form, has been found to occur within about 500 femtoseconds, that is half of one millionth of one millionth of a second. Subsequent to this initial conformational change there is a sequence of other steps, discovered using pulsed laser studies, that leads to an eventual electrical signal at the nerve cell. The electrical signal that is sent from the retina to the brain over the optic nerve is not simply the sum of all rod and cone firings. Somehow the activity of the many rod and cone interconnections "pre-processes" information about the light falling on them so that a significant part of "seeing" occurs prior to what goes on in the visual cortex of the brain.
Now that we have some idea of how the eye detects photons of light, how are colors sensed?
Rods are most sensitive (have their strongest absorption) to blue light (See this site for its first two figures: rods and cones of the eye.) But rods are not able to distinguish all of the different colors we can see in bright light. Cones are much less sensitive to light; in fact the cones effectively "turn off" in dim light. In dim light there is little distinction between colors; everything appears gray to us. In bright light, the cones take over. They have three different types of visual pigments, each having a maximum absorption corresponding to a different color. It has long been known that (almost) any light color can be represented as a sum of three "primary" light colors - red (R), green (G), and blue (B). The RGB system is the basis for color TV, for example. On a color TV screen, each pixel is divided into a R, a G and a B subpixel. Three electron beams sweep rapidly across the screen lighting each of these with a certain amount of R, of G and of B. Clearly it is tempting to explain the RGB color system in terms of the three different cone cells. Undoubtedly, there is some connection between the two, but the connection must be fairly subtle, and, as yet, is still not worked out. One reason for this situation is that the "R" cone cells actually have their response maximum closer to yellow than to red. A second reason is that some people only have two of the three cone cells, yet many of them seem to perceive color about as well as people with the normal distribution of cells. Somehow, their brains fill in the missing information. Finally, in people who are red-green color blind, all three cells appear to be present. So, the final word is not in yet.
How the eye forms images of objects out in the world will be considered later in the course after we have learned about geometrical optics.
Physical Properties of Light
Now that we have seen something about how we see light and its different visible colors, what parameter that describes light is different for different colors?
To answer this we need to first develop some basic ideas about oscillations and waves.
