Course: Combined Science + Separate Physics | Tier: Foundation + Higher (wavefronts are HT) | Linked equation: v = fλ | Linked practical: Required Practical 9: Light
Notes
What is refraction?
Refraction happens when a wave crosses a boundary between two materials and changes speed. If it hits the boundary at an angle, this makes it change direction.
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- Refraction is the change in direction of a wave when it passes from one material into another and changes speed.
- If the wave slows down (e.g. air → glass), it bends towards the normal.
- If the wave speeds up (e.g. glass → air), it bends away from the normal.
- If the wave travels along the normal, it changes speed but does not change direction.
- The frequency stays the same. The speed and wavelength change.
Drawing a refraction ray diagram
- Draw the boundary and the incident ray with an arrow.
- Draw a dashed normal at 90° to the boundary where the ray hits.
- Decide whether the wave speeds up or slows down in the new material.
- Draw the refracted ray bending towards the normal (slower) or away from it (faster).
For a rectangular glass block, the ray bends towards the normal on the way in and away from the normal on the way out. The ray leaving the block is parallel to the ray that went in.
Wavefront diagrams (HT)
A wavefront is a line joining points on a wave that are in step, such as all the crests. Wavefronts are drawn at 90° to the direction the wave is travelling.
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When a wavefront hits a boundary at an angle, one side of it enters the new material first and changes speed, while the other side is still travelling at the old speed. This makes the wavefront turn, so the wave changes direction. In the slower material the wavefronts are closer together because the wavelength is shorter.
Refraction of water waves
In a ripple tank, water waves slow down when they pass from deep to shallow water. Their wavelength gets shorter and, if they cross the boundary at an angle, they change direction.
Questions
Q1 (F) What is refraction?
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The change in direction of a wave when it passes from one material into another and changes speed.
Q2 (F) Light travels from air into glass. Does it bend towards or away from the normal?
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Towards the normal, because it slows down in glass.
Q3 (F) A ray of light enters a glass block along the normal. What happens to its direction and speed?
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Its direction does not change, but it slows down.
Q4 (F) Which property of a wave does not change when it is refracted?
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Frequency.
Q5 (F/H) Water waves move from deep water into shallow water. What happens to their speed and wavelength?
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Both decrease. The waves slow down and their wavelength gets shorter (the frequency stays the same).
Q6 (F/H) Describe the path of a ray of light that passes through a rectangular glass block at an angle.
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It bends towards the normal as it enters the glass (slows down) and bends away from the normal as it leaves (speeds up). The emerging ray is parallel to the incident ray.
Q7 (H) Light of frequency 5.0 × 1014 Hz travels at 2.0 × 108 m/s in glass. Calculate its wavelength in the glass.
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F: v = f λ
I: 2.0 × 108 = 5.0 × 1014 × λ
F: λ = 2.0 × 108 ÷ 5.0 × 1014
A: 4.0 × 10−7 m
Q8 (H) Use the idea of wavefronts to explain why light changes direction when it enters glass at an angle.
Show answer
One side of each wavefront reaches the glass first and slows down, while the other side is still in air travelling faster. The wavefront turns, so the direction of the wave changes (towards the normal). The wavefronts are closer together in the glass because the wavelength is shorter.
