Physics · Light · Reflection and refraction
Reflection and Refraction
Change the path of light with a mirror, a glass block, a water tank and a prism; measure the angles with a protractor and discover total internal reflection and the splitting of colours.
See the law of reflection and refraction by measuring angles, and find the critical angle.
20 minDifficulty: Medium
Learning goals
- Measure and compare the angles of incidence and reflection on a mirror.
- Measure how light bends going from air into glass and water.
- Find the refractive index of a medium from the ratio sin θ₁ / sin θ₂.
- Find total internal reflection and see white light split into colours in a prism.
Materials and tools
- Glass (n ≈ 1.52)
- Water (n ≈ 1.33)
- Light source (laser or white light)
- Plane mirror
- Semicircular glass block
- Semicircular water tank
- Glass prism
- Protractor (1°)
Steps
- Put the mirror on the table and measure the angles of incidence and reflection at three different angles.
- Put the glass block on the table, aim the light at the middle of its flat side at three different angles and measure the angle of refraction.
- Try the water tank: at the same angle of incidence, does water or glass bend the light more?
- Aim the light through the curved side of the glass block at its centre; find the angle at which no light gets out into the air, record it and look at the table.
Scientific explanation
Light travels in a straight line. When it hits a mirror it comes back at the same angle: the angle of incidence equals the angle of reflection.
Light slows down and bends when it enters glass or water; this is called refraction. It is why a spoon in a glass looks broken. Glass bends light more than water does.
If light leaving glass for air arrives at too steep a slant, it cannot get out and is reflected back completely. Optical fibres carry light this way. In a prism each colour bends differently and the colours of the rainbow separate.