Section 1 of 5Refraction Rules
Definition
- Refraction is the change in direction of a wave caused by a change in its speed as it crosses a boundary.
Denser medium
- When light enters a denser medium such as glass from air, it slows down and bends towards the normal.
Less dense medium
- When light leaves glass for air, it speeds up and bends away from the normal.
Along the normal
- A ray travelling along the normal does not change direction, although its speed still changes.
Section 2 of 5Investigating Refraction with a Block
Investigating Refraction with a Block
Variables
- The independent variable is the material of the block, and the dependent variable is the angle of refraction.
Controls
- The width of the light beam and the frequency of the light are kept the same for every test.
Analysis
- Plotting sin i against sin r gives a straight line whose gradient equals the refractive index of the material.
Accuracy
- A higher-resolution protractor and a sharpened pencil reduce random errors when marking the rays.
Section 3 of 5Light Passing Through a Glass Block
1
The ray bends towards the normal as it enters the glass, because light slows down.
2
Inside the glass the ray travels in a straight line at a reduced speed.
3
At the glass-to-air boundary the ray bends away from the normal as it speeds up.
4
The emerging ray is parallel to the original ray but displaced sideways.
Section 4 of 5Worked Example
Question
- A ray of light in air meets a glass block of refractive index 1.5 at an angle of incidence of 30°.
- Calculate the angle of refraction.
- Rearrange Snell's lawsin r = sin i ÷ n
- Substitutesin r = sin 30° ÷ 1.5 = 0.5 ÷ 1.5 = 0.333
- Inverse siner = sin⁻¹(0.333) = 19.5°
Answer r ≈ 19° (to 2 significant figures).
Section 5 of 5Exam Checklist & Tips
3.5.1 – Key Points for the Exam
- Refraction The change in direction of a wave caused by a change in its speed at a boundary.
- Normal A line drawn perpendicular to a boundary at the point of incidence.
- Refractive index The ratio of the speed of light in a vacuum to its speed in the medium.
- Snell's law The relationship n = sin i ÷ sin r between refractive index and the two 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.
Common Mistakes
- Saying light bends towards the normal when it leaves glass for air, when it actually bends away.
- Inverting the ratio, writing n = sin r ÷ sin i.
- Saying the frequency of light changes when it refracts.
- Using radian mode on the calculator, which gives a wrong angle.