Physics of the Visual Stimulus

In order to get a fuller understanding of why we see things as we do, it helps to learn a little about the physics of light. Light is, after all, that which enters our eyes and causes us to see.

Light is electromagnetic energy. The electromagnetic spectrum is very large ranging from gamma rays to AM waves. The visible part of this range is very small ranging from about 400 nanometers to a little over 700 nanometers.  To see an explanation of the complete electromagnetic spectrum click here.

That light may come directly from a source like a light bulb and TV screen or it may be reflected light as comes from, say, a piece of paper or from a movie screen.

Light has been considered as energy packaged in particles (the particle theory) or in waves (the wave theory) Since it is easier to diagram the idea behind the wave theory I give an illustration of the wave nature of light.

Reflecting Surfaces

As can be seen in the surfaces demo, there are three major classes of surfaces from which light can be reflected.

One surface with which we are all familiar is a mirror. A beam of light that hits a mirrored surface at a given angle will be reflected off the mirror in exactly that the same angle. See middle panel of the demo. The jargon is that the angle of reflection = the angle of incidence.  One of the interesting factors about mirrored surfaces is that they faithfully reflect images.  That is why we can use them to see ourselves.  One question that is often asked is how come there seems to be a left right reversal when looking in a miror?  Another way to ask this same question is why is text viewed through a mirror backwards?  If you would like to know the answer to this question click on reflection from mirrors.

When light hits a perfectly diffuse surface, (we often call such surfaces mat) it is reflected approximately equally in all directions. See the top panel of the demo .

Some surfaces appear neither mirror like (glossy) or perfectly diffuse (mat), they are what is frequently called semi-gloss. That is to say they exhibit highlights as well as mat areas. The bottom panel of the demo. illustrates the light reflection properties of such a surface.

The color reflected from a mirrored surface will depend only on the color of the light hitting the mirror. However, the color reflected from, say a piece of paper depends on the spectral reflectance of the paper as well as the spectral properties of the light. To see this idea graphically click on colored paper.

Transparent Surfaces

When light encounters certain materials, for example, glass and plastic, most of the light appears to pass through unhindered. These materials are called transparent. However, in fact, 100% of the light will not pass through them. Click on transparent media to see the various things that can happen to light in such materials. When light passes from one transparent medium to another it often under goes a process called refraction and obeys Snell's Law.

Polarized Light

You probably have, at one time or another, worn sunglasses that carried the Polaroid label. Polaroid, a company probably best known for its instant cameras, got its start when Edward Land discovered a new way to make polarizing filters.

If you see sunglasses that claim to be polarizing you can easily check them out for yourself. Take two pairs of sunglasses. Superimpose two of the lenses and then slowly rotate one with respect to the other. If the lenses are polarizing filters you should see the amount of light that passes through them change as a function of their relative angles. In one position almost very little will pass through (probably only some deep blue light*) and 90 degrees from this position you should see the maximum amount of light pass through. To understand this test for polarizing filters and how they work select an explanation.

The dark greenish filter we often seen in polarizing sunglasses is not the only means of polarizing light. Light can be polarized by scattering, reflection and absorption (Falk et al., 1986). Polaroid filters typically seen in sunglasses absorb one electrical field of polarized light while transmitting the orthogonal field.

For a more complete account of polarization see a very readable chapter entitled "Scattering and Polarization" by Falk, Brill and Stork (1986)

* This type of polarizing filter is not very effective for short wavelength lights. Therefore these wavelengths do not become very polarized and pass through the crossed filters even when other wave lengths don't.

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