# Q & A: Prisms Make Colorful Light

Q:
How does a prism work?
- chase gilbert (age 9)
Elbow Valley Elementary, Calgary Alberta Canada
A:
When light goes into a piece of glass it bends, unless it goes straight into the surface. (You've seen something just like this when you look at things under water from above the water. ) Now the interesting thing is that different colors of light bend different amounts. (I'll explain why below in case your interested.) You don't notice that because in a window the light bends back on the way out, so the different colors are just barely shifted from another. Because the prism faces aren't parallel, the bending on the way out doesn't just reverse the bending on the way in. So different colors of light come out at different angles, and gradually spread apart. Since a beam of white light is actually made up of all the colors, you can see the different colors because they all come out moving in different directions.

Light going through lenses does the same thing a little bit, causing a problem called "chromatic aberration". The different colors focus at slightly different points because they bend different amounts.

Why do different colors bend different amounts? The amount the light bends depends on how much it slows down in the glass. How much it slows down depends on how much its electromagnetic field shakes the electrons in the glass. The electrons respond a little differently to different frequencies of shaking, and different frequencies of light means the same thing as different colors.

Mike

(published on 10/22/2007)

## Follow-Up #1: Does gravitational lensing have aberrations?

Q:
I was thinking if gravitational lens causes "chromatic aberration" by 1)Newtonian mechanics and 2)General relativity and tentatively concluded that 1)says-Yes 2)says-No. so, what does nature(experimental data) say? No? Do you get the same results by 1) and 2)? I want to make sure I did the right thought experiments.
- Anonymous
A:

I'm not sure what the difference between Newtonian and relativistic gravitational lensing is. Perhaps you can elaborate on your thought experiments?

In nature, gravitational lensing does not contain chromatic aberration (like dispersion), since all wavelengths of light are bent by the same amount. This can be seen by the equivalence principle. I like to remember that gravitational fields are equivalent to accelerating reference frames, and use the elevator thought experiment:

As for the other aberrations, all I can say is that they must be very large (in general). The matter making up the "lens" is not smoothly distributed, so the lens will be far from perfect.

I know a fair amount about lens aberrations, but very little about gravitational lensing. I'm curious whether knowledge of optical aberrations is useful for calculating or understanding gravitational lensing (I'm guessing probably not)... maybe someone else can answer that question.

David Schmid

(published on 04/21/2013)