5.3.1Waves
Refraction of Light — SQA National 5 Physics Revision Notes & Topical Questions
5 min read 5 parts Lesson 7 of 8
Part 1 of 5What Refraction Is
Refraction explains why a straw looks bent in a glass of water and why swimming pools look shallower than they are.
Definition
- Refraction is the change in direction of a ray of light when it passes from one transparent material into another of different density.
Cause
- Light travels at different speeds in different materials, and it is this change of speed at the boundary that causes the bending.
The normal
- A dashed line drawn at 90° to the boundary;
- all angles of incidence and refraction are measured from it, not from the surface.
Along the normal
- A ray travelling along the normal (angle of incidence 0°) crosses the boundary without changing direction, though its speed still changes.
Partial reflection
- At the boundary some light is also reflected, but refraction concerns the part that passes into the new material.
Part 2 of 5Direction of Bending
Direction of Bending
Air to glass
- Light slows down and bends towards the normal, so the angle of refraction is smaller than the angle of incidence.
Glass to air
- Light speeds up and bends away from the normal, so the angle of refraction is larger than the angle of incidence.
Slower medium
- Whenever light enters a material in which it travels more slowly, it bends towards the normal.
Faster medium
- Whenever light enters a material in which it travels faster, it bends away from the normal.
Colours
- Different colours slow by slightly different amounts, so white light can be split into a spectrum by a prism.
| Property | Air to glass | Glass to air |
|---|---|---|
| Speed | Decreases | Increases |
| Wavelength | Decreases | Increases |
| Frequency | Unchanged | Unchanged |
| Bends | Towards normal | Away from normal |
Part 3 of 5What Changes at a Boundary
1
The frequency of the light is set by the source, so it stays the same as the wave crosses the boundary.
2
The speed changes because the optical density of the new material is different, so light travels more slowly in glass than in air.
3
Since v = fλ and f is fixed, a change in speed forces the wavelength to change in the same proportion.
4
The wavefronts hit the boundary at an angle, so one side slows before the other, which turns the direction of travel.
5
The light is said to refract, and its path bends towards or away from the normal depending on the change in speed.
Part 4 of 5Worked Example
Question
- Red light of frequency 4.5 × 10¹⁴ Hz travels in air at 3.0 × 10⁸ m/s and enters glass, where its speed is 2.0 × 10⁸ m/s.
- Calculate its wavelength in air and in glass.
- In airλ = v ÷ f = 3.0 × 10⁸ ÷ 4.5 × 10¹⁴ = 6.7 × 10⁻⁷ m.
- The frequency stays 4.5 × 10¹⁴ Hz in glass.
- In glassλ = 2.0 × 10⁸ ÷ 4.5 × 10¹⁴ = 4.4 × 10⁻⁷ m.
- The wavelength decreases, which agrees with the light slowing down.
Answer λ(air) ≈ 6.7 × 10⁻⁷ m and λ(glass) ≈ 4.4 × 10⁻⁷ m.
Part 5 of 5Exam Checklist & Tips
5.3.1 – Key Points for the Exam
- Refraction The change in direction of a wave when its speed changes at a boundary.
- Normal A line drawn at right angles to a boundary, used to measure angles.
- Angle of incidence The angle between the incident ray and the normal.
- Angle of refraction The angle between the refracted ray and the normal.
- Optical density A measure of how much a material slows down light compared with air.
Common Mistakes
- Measuring angles from the surface of the block instead of from the normal.
- Saying the frequency of light changes when it enters glass; only speed and wavelength change.
- Drawing the ray bending away from the normal when entering glass from air.
- Forgetting that a ray hitting the boundary along the normal does not change direction.
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