When we look at the image we see a range of wavelengths from 0.1 Å (1 ångström = 10-10 meter = 0.1 nanometer) to 1000m. As you can see you have encountered all of these wavelengths before, be it at the doctor’s office when you broke your leg, your kitchen when you heated up a meal, in your living room when you watched your favorite show or in your car when you listened to the radio. When discussing color we are mainly concerned with the range of wavelengths between 400nm and 700nm: the visible or optical spectrum. It is this range of energy that humans can perceive as color. Higher energy levels than the visible part include ultraviolet, X-ray and gamma radiation. Lower energy levels include infra-red, microwave, short wave, and long wave radiation.
When we look at the visible part of the spectrum we can see that electromagnetic energy with a wavelength of 400nm represents violet light, 520nm lets us see green, 590nm yellow and 700nm results in red. When all wavelengths of the visible spectrum reach our eye simultaneously we perceive that light as being white; we do not see any specific color, just white light.
It is important to mention the terms additive and subtractive color mixing at this point. Unlike with mixing pigments, which is subtractive color mixing, the transmitting of certain colors by a gemstone results in additive color mixing. Let’s take a look at the two images below.
Additive Color Mixing; a Red, Green, and Blue Flashlight Pointed at a Wall, where there is Overlap, we See White.
Subtractive Color Mixing; a Yellow, Magenta, and Cyan Paint on a White Piece of Paper, where they Overlap we See a Very Dark Gray.
The computer screen you are looking at right now is emitting three different colors per pixel: red, green and blue. The relative intensity of each of these three colors will determine which color you see. If you take a loupe or magnifying glass and hold it up to the screen you see how it works: in the white areas of your screen the three colors are emitted in equal intensity. The text, which you perceive as black, is composed of the three colors emitting hardly anything. On the left and right, where the light green is going over into the darker green you can see a difference in the intensity of the red color pixel. In the dark green the red is clearly less bright. This additive color mixing of different colors of light is similar to what happens when you are looking at the color of a gemstone.
It is when certain wavelengths are reaching your eye in higher intensities than others that we see a certain hue. This is where gemstones get their color or lack thereof. Some colors (read: wavelengths) are being absorbed while others are transmitted. What color we see depends on the combination and intensities of the different colors that are transmitted.
Spectrum on Spessartite from Loliondo, Tanzania.