Part 09 · Induction, light and optics · Card 8 of 11
Snell's law
When light crosses between materials its direction bends: n1 sin theta1 = n2 sin theta2.
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Analogy
A car turning when one wheel hits mud first.
Example
Air to glass
1 x sin 30 = 1.5 x sin theta2sin theta2 = 0.333theta2 = 19.5 degrees
In practice
Light entering glass (n = 1.5) from air at 30 degrees bends to about 19.5 degrees.
Practice
Flashcards
- State Snell's law.n1 sin theta1 = n2 sin theta2.
Interview questions
- Light enters a glass block at 40 degrees. The glass has refractive index 1.5. Find the angle in the glass and explain total internal reflection.
- Use Snell's law
- sin theta2 = sin 40 / 1.5
- Critical angle for glass to air is 41.8 degrees
Questions people ask
What is the refractive index?
The refractive index n is the ratio of the speed of light in a vacuum to the speed in the material, n = c/v. Water has n of 1.33 and glass about 1.5. A higher n means slower light and more bending toward the normal.
Why does a straw look bent in water?
Light from the part under water bends as it leaves into air, so rays seem to come from a different place. Your brain traces them straight back, making the submerged part look raised and the straw look broken at the surface.