Total internal reflection
Light going from a dense to a less dense material reflects completely beyond the critical angle, sin theta c = n2/n1.
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Analogy
The reason diamonds sparkle and optical fibres carry signals.
Example
sin theta c = 1 / 1.5= 0.667theta c = 41.8 degrees
In practice
For glass to air the critical angle is about 41.8 degrees.
Practice
Flashcards
- When does total internal reflection happen?When light goes from higher to lower refractive index beyond the critical angle.
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
How does an optical fibre work?
Light enters a thin glass core and strikes the core wall beyond the critical angle, so it reflects completely each time. It zig-zags along the fibre with almost no loss for kilometres. Fibres carry internet data as pulses of light.
Why can't total internal reflection happen from air to glass?
It needs light to move from a material with higher refractive index to a lower one. Going from air (n = 1) to glass (n = 1.5), the bent ray always exists, because sin theta2 would be less than 1. There is no critical angle in that direction.