Total internal reflection and optical fibres
Send a ray through a semicircular glass block, increase the angle until it can no longer escape, find the critical angle, and see how optical fibres trap light.
Core concept
Light inside a denser medium is totally reflected at the surface when it meets it beyond the critical angle
Topic: Light
When light goes from glass into air it bends away from the normal. As the angle of incidence inside the glass increases, the refracted ray bends further until, at the critical angle, it skims along the surface (angle of refraction: When light bends as it passes from one material into another, like from air into water. All key words 90°). Beyond the critical angle no light can escape: all of it is reflected back inside. This is total internal reflection: When light bounces off a surface, like a mirror. All key words. For a material with refractive index n, sin c = 1 ÷ n. Optical fibres use total internal reflection to carry light, and the internet data it encodes, along thin glass threads.
What you will learn
- Describe what happens to a ray as the angle of incidence inside a block increases.
- Find the critical angle and relate it to refractive index (sin c = 1 ÷ n).
- Explain how optical fibres carry light.
Before you start
- Know that light bends away from the normal leaving a denser medium.
Materials
- Semicircular glass block
Equipment
- Ray box, protractor and paper
- Optical fibre or a model made from a bent stream of water
Procedure
- With the glass block, shine the ray at 30° and record what happens.
- Increase the angle step by step to find the critical angle, recording just below and just above it.
- Change to the optical fibre view and send the ray at 60°, then at 30°.
- Try water and diamond and compare their critical angles.
How this simulation is simplified
- Refractive indices: glass 1.50, water 1.33, diamond 2.42; the block is surrounded by air.