Physics, visually · Waves, Sound & Light · Concept 10
Reflection and Refraction
Why does a straw look bent in water, and how does light stay trapped inside an optical fibre?
Light bounces off mirrors at a mirror angle, and bends when it changes material. Tilt the beam and see the exact rule.
- Mirror tab. Change the angle of incidence. The reflected beam always leaves at the same angle on the other side of the normal.
- Bending tab. Choose Air → Glass. The beam bends towards the normal. Then Glass → Air: it bends away.
- Choose Glass → Air and raise the angle past about 42°. The beam stops leaving: total internal reflection.
Slow side, squeeze in: bend toward the normal.
Reflection: angle in = angle out, both measured from the normal (the line at 90° to the surface). Refraction: light slows in glass or water, so it bends towards the normal; leaving, it bends away. Past the critical angle it cannot leave at all.
Optical fibres and light guides
Light is kept inside the fibre by total internal reflection. Fibre-optic sensors and data links rely on it.
Photoelectric and laser sensors
A reflective sensor bounces a beam off a mirror (retro-reflector) and checks that the beam returns. Mirror alignment means angle in = angle out.
Lenses on vision cameras
Machine-vision lenses bend light by refraction to focus the part onto the sensor. Different glass changes the focus.