HOW LITTLE WE SEE!

To get some idea of what a small EM “window” is represented by the visiblelight wavelengths, try looking through a red- or blue-colored piece of glass or cellophane. Such a color filter greatly restricts the view you get of the world because only a narrow range of visible wavelengths can pass through it. Different colors cannot be ascertained through the filter. For example, when a scene is viewed through a red filter, everything is a shade of red or nearly red. Blue appears the same as black, bright red appears the same as white, and maroon red appears the same as gray. Other colors look red with varying degrees of saturation, but there is little or no variation in the hue. If our eyes had built-in red color filters, we would be pretty much color-blind.

When considered with respect to the entire EM spectrum, all optical instruments suffer from the same sort of handicap we would have if the lenses in our eyeballs were tinted red. The range of wavelengths we can detect with our eyes is approximately 770 nm at the longest and 390 nm at the shortest. Energy at the longest visible wavelengths appears red to our eyes, and energy at the shortest visible wavelengths appears violet. The intervening wavelengths show up as orange, yellow, green, blue, and indigo.