3.3 Refraction
Summary: You can bend a pencil without touching it. Put it in a glass and fill it with water. It looks bent, but it isn’t. Why? Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- explain what happens when Light changes direction
- describe how a convex lens affects light.
Think back
- What is a normal on a ray diagram?
- What is a virtual image?
- What is the law of reflection?
Key idea Refraction changes the direction that a ray of light travels in. Lenses focus light.
Key words refraction, medium, parallel, lens, convex, converging, focus, focal point, focal length
You can bend a pencil without touching it. Put it in a glass and fill it with water. It looks bent, but it isn’t. Why?
Why does a pencil look bent in water?
The pencil reflects light. The light travels from the pencil through the water. It then travels through the air into your eye. As the light leaves the water, the direction it is travelling in changes. This is called refraction.
Refraction happens when light travels from one medium (material) to another. The change in direction explains why the pencil looks bent. You can see this in Figure 1.
Figure 1Pencils look bent when you put them in water.
A Explain why the image of the pencil in the water is virtual. The brain thinks that the light has travelled in a straight line. This means that the end of the pencil seems to be in a different place to where it actually is. The pencil looks bent. Refraction also explains why a swimming pool looks less deep than it actually is, like in Figure 2.
Figure 2A rock at the bottom of a pool looks closer to the surface than it actually is. Full text alternative to the content
Why does light change direction?
Imagine a truck driving from a road onto mud, like in Figure 3. When the first wheel of the truck hits the mud, it slows down. The other wheels keep going. This changes the truck’s direction. This is similar to what happens when light travels from air into water or glass, or when water waves go from deep to shallow (less deep).
B Describe the difference between reflection and refraction.
Figure 3Like a truck that changes direction when it slows down, light and waves get refracted when they slow down. Full text alternative to the content
When light enters a glass block, it slows down. When it comes out, it speeds up again. Its direction changes twice. Light bends towards the normal when it goes into the glass. It bends away from the normal when it comes out. The rays going into the block and the rays coming out are parallel (the same distance apart at every point).
What does a lens do?
There are two lenses in your body. The lenses in your eyes are convex or converging lenses. They focus the light and allow you to see. The light is refracted as it goes into the lens and as it comes back out. Figure 4 shows how a lens focuses light.
Figure 4A piece of glass shaped like a lens focuses light. Full text alternative to the content
C Describe what happens to the Speed of the light when it comes out of the lens.
Literacy skills In each list below, choose the correct spelling of the word. Make up a rule that will help you remember the spelling. a) lense, lens, lenz b) parallel, parralell, paralell
Summary questions
- Copy and complete the sentences. When someone looks at a rock at the bottom of a swimming pool, it appears above/below where it actually is. This is because the light reflects/refracts when it travels from water into air. It bends towards/away from the normal as it goes into the air. This is because it slows down/speeds up. A convex/plane lens focuses light.
- Explain what would happen to: the speed and direction of light waves if they went straight into shallow water rather than at an anglethe speed and direction of light waves if they went straight into a glass block rather than at an angle.
- Suggest what happens to the focal length when you make a lens thinner.