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Unlock This CourseRemember, refraction is the change in direction of a wave when it crosses a boundary and its speed changes. You met it with water waves in Describing Waves. Light does exactly the same thing.
Light travels very fast in air. In a transparent material such as glass or water it travels more slowly. So when a ray of light crosses the boundary between air and glass, its speed changes. If the ray meets the boundary at an angle, its direction changes too. The ray is straight in the air and straight in the glass, with one sharp bend exactly at the boundary.
We describe the bend using the normal, the dashed line at 90° to the boundary that you used in Reflection of Light. The ray arriving at the boundary is still called the incident ray, and the angle of incidence is still measured from the normal.
Key terms:
There are only three rules to learn. They work for glass, water and clear plastic.
| The ray is going... | Speed of the light | What the ray does | Angles |
|---|---|---|---|
| from air into glass or water, at an angle | slows down | bends towards the normal | angle of refraction is smaller than angle of incidence |
| from glass or water into air, at an angle | speeds up | bends away from the normal | angle of refraction is bigger than angle of incidence |
| along the normal (at 90° to the boundary) | still changes | does not bend, it carries straight on | both angles are 0° |
Look at the last row carefully. A ray along the normal still slows down when it enters glass. It just does not change direction.
The numbers below show what happens for one common type of glass. You do not need to learn them. Notice the pattern: unless the ray is along the normal, the angle of refraction is smaller than the angle of incidence, and a bigger angle of incidence gives a bigger angle of refraction.
| Angle of incidence in air | Angle of refraction in glass |
|---|---|
| 0° | 0° |
| 20° | about 13° |
| 40° | about 25° |
| 60° | about 35° |
One suggested way to model refraction is to roll a toy car from a smooth floor into a region of sand. This is an optional extra, not something you have to do.
One front wheel reaches the sand first and slows down. The other front wheel is still on the smooth floor, so it keeps going at the faster speed. For a moment the two sides of the car travel at different speeds, so the car turns. It turns towards the normal, just as light does going into glass.
If the car meets the edge of the sand head on, both front wheels reach the sand at the same time. They slow down together, so the car gets slower but does not turn. This matches a ray of light travelling along the normal.
The model also works backwards. A car rolling out of the sand at an angle has one wheel that speeds up first, so the car turns away from the normal.
This is the refraction diagram you are most likely to be asked to draw or complete. The ray bends twice: once going in and once coming out. Use a sharp pencil and a ruler.
The emergent ray is parallel to the incident ray because the two edges of the block are parallel. The bend away from the normal on the way out exactly undoes the bend towards the normal on the way in. So the light ends up travelling in its original direction, but shifted sideways.
Question: A ray of light in air meets the top of a rectangular glass block with an angle of incidence of 40°. The angle of refraction in the glass is 25°. State the angle between the emergent ray and the normal at the bottom edge, and give a reason.
Answer: 40°. The edges of the block are parallel, so the emergent ray is parallel to the incident ray. It makes the same angle with the normal as the incident ray did.
Inside the glass, the ray meets the bottom edge at 25° to the normal, then bends away from the normal to 40° as it speeds up in the air.
Your brain assumes that light always reaches your eyes in a straight line. When light has been refracted on the way, objects seem to be in the wrong place.
Light from the part of the straw under the water bends away from the normal as it leaves the water. Light from the part above the water does not cross that boundary. So the bottom half seems to be in a different place from the top half, and the straw looks bent or broken at the surface.
Light from the bottom of a swimming pool bends away from the normal as it leaves the water. Your brain traces the rays straight back, to a point higher up than the real bottom. So the pool looks shallower than it really is.
A ray of light travels through air and meets the side of a rectangular glass block. The angle of incidence is 50°.
(a) State what happens to the speed of the light as it enters the glass. [1 mark]
(b) Describe how the direction of the ray changes as it enters the glass. [2 marks]
(c) The ray leaves the block through the opposite side. State the angle between the emergent ray and the normal, and give a reason for your answer. [2 marks]
(d) A diagram shows the block and the incident ray. Describe how to complete the diagram to show the path of the ray through the block and out into the air. [4 marks]
(e) A second ray meets the block along the normal. Describe the path of this ray as it enters the glass. [1 mark]
(a) It decreases, or the light slows down (1).
(b) The ray bends towards the normal (1), so the angle of refraction is smaller than 50°, the angle of incidence (1).
(c) 50° (1), because the emergent ray is parallel to the incident ray (1).
(d) Draw a dashed normal at 90° to the side where the incident ray meets the block (1). Rule the refracted ray inside the block, closer to the normal than the incident ray (1). Draw a second normal where this ray meets the opposite side (1). Rule the emergent ray bending away from the normal, parallel to the incident ray, with arrows on the rays (1).
(e) It carries straight on without changing direction (1).
In part (b), 'it bends' is not enough for the mark. Always say which way: towards the normal or away from the normal. Before you draw anything in a question like part (d), put in the dashed normals first, because every bend is judged against them.