Trapped light and spread colours

Science · Waves, Light & Sound · ages 19-20

Name ______________________ Date ____________

Total internal reflection can happen going from air into water.
Circle one: True False
What two conditions give total internal reflection?
- Rare to dense with a steep hit
- Dense to rare past the critical angle
- Any boundary at any angle
How is the critical angle found from n1 dense and n2 rare?
- Sine equals n1 over n2
- Sine equals n2 over n1
- Angle equals n1 minus n2
Which pair traps light most steeply, with the smallest critical angle?
- Water to air at 48.6 degrees
- Polystyrene to air at 42.2 degrees
- Diamond to air at 24.4 degrees
A boundary has n2 over n1 equal to 1 over 2. Give the sine of the critical angle.
Answer: ______________
Why does a fibre need lower-index cladding?
- To keep the core boundary trapping
- To absorb stray colours
- To magnify the signal
Critical angle is 42.2 degrees. A ray hits exactly at it. Give the refracted angle in degrees.
Answer: ______________
Fibre pulses smear because faster colours leak out of the core.
Circle one: True False

Answer key

For grown-ups. Fold this page away before handing over the rest.

False · Air to water is rare to dense, so escape stays possible.
Dense to rare past the critical angle · Only dense to rare past the critical angle traps all light.
Sine equals n2 over n1 · Snell with 90 degrees on the rare side gives n2 over n1.
Diamond to air at 24.4 degrees · Stronger contrast gives a smaller critical angle.
0.5 · Sine of the critical angle is exactly that ratio.
To keep the core boundary trapping · Cladding keeps the index step that reflection needs.
90 · At the critical angle the refracted ray runs along the surface.
False · Smearing is delay spread, while the light stays trapped.