Section 1 of 7Wavefronts (HT only)
What a wavefront is
- What a wavefront is A wavefront is a line joining points on a wave that are in step, for example all the crests;
- it is drawn at right angles to the direction of travel.
- Rays show direction; wavefronts show the shape of the wave.
Spacing
- Spacing The gap between neighbouring wavefronts equals one wavelength.
Section 2 of 7Why a wave bends (HT only)
1Part of the wavefront arrives first
- Part of the wavefront arrives first When a wave hits a boundary at an angle, one end of each wavefront enters the new medium before the other end.
2That part changes speed
- That part changes speed The end in the new medium slows down (or speeds up) while the end still in the old medium keeps its original speed.
3Wavefront pivots
- Wavefront pivots The two ends travel different distances in the same time, so the wavefront swings round.
4Direction changes
- Direction changes Since the ray is at right angles to the wavefront, the ray bends.
Section 3 of 7Slowing down and speeding up (HT only)
| Situation | Wavefront spacing | Ray direction |
|---|---|---|
| Slows on entering new medium | Closer together (shorter wavelength) | Bends towards the normal |
| Speeds up on entering new medium | Further apart (longer wavelength) | Bends away from the normal |
| Hits boundary head-on | Spacing changes but no pivot | No change in direction |
Section 4 of 7Explaining it in an answer (HT only)
1
State that different parts of the wavefront reach the boundary at different times.
2
State that the part in the new medium changes speed while the rest does not.
3
Say the wavefront therefore changes direction, drawing the ray at right angles to it.
4
Link speed to wavelength with v = f × λ, with frequency constant, to explain the closer or wider spacing.
5
Conclude: a change in speed causes the change in direction.
Section 5 of 7Example: water waves
Deep to shallow
- Deep to shallow Ripples slow in shallow water; wavefronts bunch closer together and bend towards the normal.
Shallow to deep
- Shallow to deep Waves speed up and spread out, bending away from the normal.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Wavefront | A line joining points of a wave that are at the same stage of their oscillation. |
| Refraction | The change in direction of a wave caused by a change in its speed. |
| Wavelength | The distance between successive wavefronts. |
| Frequency | The number of wavefronts passing a point per second; it stays constant when a wave refracts. |
| Ray | A line showing the direction of travel of a wave. |
Section 7 of 7Exam technique
1
Explaining refraction as 'light being attracted' to the normal: the cause is a change in speed.
2
Drawing wavefronts parallel to the raywavefronts are at right angles to it.
3
Forgetting that wavelength changes when speed changes while frequency stays constant.
4
Saying a wave slows down at a boundary only because of its direction change: the speed change happens first and causes the bending.