There are two basic ways by which we can see colors. Either an object can directly emit light waves in the frequency of the observed color (a neon light does this), or an object can absorb all other frequencies, reflecting back to your eye only the light wave, or combination of light waves, that appears as the observed color (any painted object does this). For example, to see a yellow object, either the object is directly emitting light waves in the yellow frequency, or it is absorbing the blue part of the spectrum and reflecting the red and green parts back to your eye, which perceives the combined frequencies as yellow.
When we talk about light in reference to sunglasses, three types of light are important.
- Direct light - Direct light is light that goes straight from the light source (like the sun) to your eyes. Too much direct light can wash out the details of your surroundings and make it almost painful to try to focus your vision on anything.
- Reflected light - Reflected light, usually in the form of glare (see explanation above), is light that has bounced off a reflective object to enter your eyes. Just like direct light, strong reflected light can make it difficult to perceive the details or directly view an object. Snow, water, glass and white sand are all good reflectors.
- Ambient light - Ambient light is light that has bounced and scattered in many directions so that it is does not seem to have a specific source. A good example of ambient light is the glow in the sky around a major city. It would be very hard to identify a single source of light for that glow. Ambient light is how you are able to see when there is no direct source of light.
Good sunglasses can eliminate the ultraviolet part of the spectrum, cut down on direct light to the point where it is comfortable and eliminate or decrease reflected light (depending on the reflecting surface).
One interesting property of light is polarization. When reflected off of certain surfaces -- such as water -- light gets polarized. Polarized sunglasses can eliminate reflected light off of water and similar surfaces because of its polarization. See the section on sunglass technologies for details.