Now we're gonna start to look at different colors to the right. You'll notice that all the simulations now have dark backgrounds. This is because when you mix color with light, it gets brighter, and harder to see on a white background. This is called additive blending.*
To the right you'll see three circles. These each represent spot lights shining on your screen. You can change each ones strength to see how the colors mix.
We're using red, green, and blue because they are the primary colors for light. You can make any other color with them. Throughout these other simulations, we'll stick with just red, green, and blue, so that it can be simulated relatively quickly. In reality, color is on a continuous spectrum from red to violet.*
On the previous page, you learned that each material has an index of refraction that determines how much light is bent. But that was only for light of one color. In reality, the index of refraction for a material varies with the color of the light. This is called dispersion.
To the right, there is a prism, with different indices of refraction for red, green, and blue. You can see how it splits the white beam into it's three colors. In reality, it would split into a continous rainbow, but we're only showing three colors because of how the simulator is programmed.
It is important to note that all conventional materials have a maximum index of refraction for red light, and the IOR steadily decreases as you reach blue light. But in this simulation, you can play around with what it would be like if the glass had a higher IOR for blue light than it did for red light.
Dispersion causes issues with lenses. As you can see to the right, each color of light is focused onto a different point. This causes an issue called chromatic aberration, which can make images taken through a lens appear distorted.
Chromatic aberration can be overcome by using a lens with a more constant IOR, or by using thin, small lenses. For everyday applications of lenses, like cameras and small telescopes, the amount of aberration is too small to matter, or can be corrected with software.
For large telescopes, though, the aberration that would be caused by the large lens would render any pictures unusable. Instead of overcoming this problem with advancements in lenses, large telescopes simply use mirrors to focus light instead.*
While focusing light with mirrors may save you from chromatic aberration, mirrors still have a problem with color. They reflect more of some colors, and less of others. You can try this by changing the reflectance of the mirror for each individual color.
Most mirrors tend to reflect more green light than anything else. This is why if you stand between two mirrors, and look into the distance, the image slowly fades into a green mist. This is also why some people will answer green when asked what color a mirror is.