What is refraction?
Refraction is the change in the direction of a wave as it passes from one medium to another where its speed changes. In a ripple tank, water waves slow down as they move from deep water into shallow water, causing them to bend.
What changes and what stays the same
When a wave refracts:
- The frequency stays constant — it is fixed by the source.
- The speed changes (slower in shallow water, faster in deep water).
- The wavelength changes in the same way as the speed (since v = fλ and f is fixed).
- The direction changes only if the wave hits the boundary at an angle.
Key formula / Key formula
Since v = fλ and frequency f is constant:
v₁/v₂ = λ₁/λ₂
Slower speed → shorter wavelength.
Worked example
Water waves of frequency 4 Hz travel at 0.20 m s⁻¹ in deep water. On entering shallow water the speed drops to 0.12 m s⁻¹. Find the wavelength in each region.
Deep: λ₁ = v₁/f = 0.20/4 = 0.05 m.
Shallow: λ₂ = v₂/f = 0.12/4 = 0.03 m.
The frequency stays 4 Hz; the wave slows and its wavelength shortens.
Direction of bending
When a wave slows down (deep to shallow), it bends towards the normal. When it speeds up (shallow to deep), it bends away from the normal. This is why light bends when it enters glass or water, and why a straw looks bent in a glass of water.
Remember
- Frequency never changes during refraction.
- Speed and wavelength change together (v = fλ).
- Slowing down → bends towards the normal; speeding up → bends away.