Physics 06253.2 Light
Refraction and total internal reflection
Shine a ray into a glass block, vary the angle of incidence and find the critical angle for yourself.
- n = sin i / sin r
- Critical angle
- Total internal reflection
Angle of incidence
35°
Angle of refraction
22.5°
Critical angle
41.8°
sin i / sin r
1.5
Predict
Going from glass into air, what happens once the angle of incidence exceeds the critical angle?
Experiment
- 1Set the direction to air → glass and confirm the ray bends towards the normal.
- 2Switch to glass → air and raise the angle slowly until the refracted ray disappears. Compare that angle with the critical angle readout.
- 3Increase the refractive index and see what happens to the critical angle.
Explain
Light slows down entering a denser medium, so it bends towards the normal. Leaving a denser medium it speeds up and bends away from the normal.
n = sin i / sin r. The critical angle is the angle of incidence in the denser medium that gives an angle of refraction of exactly 90°. Past it, refraction is impossible and every ray is reflected — which is how optical fibres carry light around bends without losing it.