Mirrors and Reflection

Light that bounces

You already know that light travels in straight lines and that it comes from sources like the Sun and lamps. But what happens when that light hits something?

Sometimes light passes through, like through a clear window. Sometimes light is blocked, which makes a shadow. And sometimes light bounces off. When light bounces off a surface, we call it reflection.

Light bounces off almost everything — that is how you see objects at all. Light from a lamp hits your book, bounces off, and travels into your eyes. But most things scatter the light in all directions, so you do not see a picture in them. A mirror is special.

Why mirrors are special

A mirror has a very, very smooth and shiny surface. Because it is so smooth, the light bounces off it in a neat, orderly way instead of being scattered.

Imagine throwing a ball at a smooth wall — it bounces straight back in a predictable way. Now imagine throwing it at a bumpy pile of rocks — it shoots off in a random direction. A mirror is like the smooth wall for light, and a rough surface like a brick is like the bumpy rocks.

Because the light bounces evenly, all the rays keep their pattern. So when light from your face hits the mirror and bounces into your eyes, you see a clear image of your face.

The law of reflection

Here is the important science rule, called the law of reflection:

Light bounces off a mirror at the same angle it hit it.

Think of a ball bouncing off the floor. If you throw it down gently at a slant, it bounces up at the same slant on the other side. Light does exactly the same on a mirror.

This is why you can aim reflections. If you tilt a mirror, you change the angle the light hits it, so you change where the light bounces to. People use this idea in periscopes (to see over walls), telescopes, and even in lasers.

The flipped image

Look at yourself in a mirror and raise your right hand. The person in the mirror raises the hand on your right side — but for them it looks like a left hand! Mirror images are swapped left-to-right.

The clearest way to see this is with writing. Hold up a book to a mirror. The words look backwards! Letters that were on the left now appear on the right. That is why ambulances sometimes write "AMBULANCE" backwards on the front — so drivers in front read it the right way round in their mirrors.

The image is not flipped up-to-down, though. Your head still looks like it is at the top. Only left and right are swapped.

Reflectors all around

Not all reflections need a perfect mirror. Many things reflect light a little:

• Still water in a pond can act like a mirror and show the sky.
• A polished metal spoon reflects your face (curved spoons make it look funny!).
• Shiny shoes, glass, and a calm puddle all reflect.

The smoother and shinier the surface, the clearer the reflection. The rougher and duller it is, the more the light scatters.

Try it yourself: bounce a light beam

Let's prove the law of reflection.

You will need: a small flat mirror, a torch, and a darkish room.

1. Stand the mirror upright on a table (lean it against some books).
2. Shine the torch at the mirror from one side, at a slant.
3. Watch where the light beam lands on the wall. Notice it bounces off to the other side.
4. Now change the angle of your torch. The bright spot moves! The more slanted your beam, the more slanted the bounce.
5. Try aiming the bounced light onto a chosen object, like a teddy. Can you "deliver" the light by tilting the mirror?

Stay safe: never shine the torch or any reflected light into anyone's eyes, and use a flat mirror with smooth edges (a parent can tape the edges).

What we learned

Reflection is light bouncing off a surface. Mirrors are smooth and shiny, so they reflect light evenly and give a clear, flipped image. Light always bounces off at the same angle it arrived — the law of reflection. Now you know why your reflection waves with the "wrong" hand!

Want to know more about how light behaves? Read Light and Shadows, and discover the colours hidden inside white light in How Rainbows Form.