How flat mirrors work
Do you like to look at yourself in a mirror? Some people do. When a person looks in a mirror they can see a picture of themselves. This picture is usually called a reflection. In science it is also called an image. What happens to let you see an image of your face in a mirror? The answer is to do with the way light rays behave at the mirror's surface.

There are certain objects which give out light. They are called light sources. Although a mirror may shine, it is not a light source. It shines because of the light that it reflects. When you look in a mirror you see yourself by light rays which have made the following journey. The light ray's journey begins at a light source. This may be the Sun or an electric lamp. It could even be a candle flame. After the light has left the source, it travels through the air and strikes your face. The light rays then bounce off your face and travel towards the mirror. When they reach the mirror's surface they bounce off again and travel into your eyes.

When light rays bounce off the surface of an object we say that they are reflected. The rays, which are reflected from the mirror and enter your eye, appear to be coming from a picture or image of your face inside the mirror. If you look in a mirror then move your face closer to it, the image of your face becomes larger and appears to be nearer the surface of the mirror. If you look in a mirror and move your face further away, the image of your face becomes smaller and appears to be further away from the surface of the mirror. In fact your image seems to be as near the surface of the mirror as you are but on the other side. If you look behind the mirror there is nothing there because the image only appears to be there. It is not really there.

You may think that your image in a mirror is an accurate picture of you but you would be wrong. Just look in a mirror and wink your left eye. What happens to the image? The right eye winks. Scratch your left cheek with a finger of your left hand and your image shows its right hand scratching its right cheek. The image of your face is the opposite way round to how your face really is. The way a flat mirror turns an image round can be strikingly seen by this simple experiment. Just open a book and turn the page towards the mirror. You will see in the reflection of the page that the words and sentences appear back to front and are very difficult to read.

When you look in a mirror you do not just see your face. You also see objects behind you. If you hold up a small mirror and look into it then turn the mirror so the left hand part points away from you, you can see other objects behind you which are far to your left. Even though you are no longer looking directly into the mirror there are still plenty of things to see. If you turn the mirror to the right a similar thing happens. You can see these objects because of the way the light behaves when it strikes the mirror at an angle.

The best way to see how light rays strike a flat mirror is to make a single light ray. This is easy to do. Just cut a small slit in a piece of card and put the card over the front of a torch. Shine the torch across a sheet of white paper to a flat mirror and see the path the light ray takes. It moves in a straight line to the mirror and strikes it at a certain angle then is reflected at the same angle but in the opposite direction and moves away in a straight line. It does not matter at what angle you shine the light ray at the mirror, the reflected ray always leaves the mirror at the identical angle at which the ray approached the mirror.

Next time you look in a mirror, bob your head close to it then bring your head back, wink and scratch your cheek and look to your left and right to increase the view behind you. Do not be surprised to see some puzzled faces as people wonder what you are doing. Just tell them you are investigating how light is reflected from a flat mirror.

Is it possible to see how you really look using flat mirrors?
Yes. If you put two flat mirrors together as if they were the pages in a book then open them and look into them, you will see perhaps four images. Tap your left cheek with a finger and look at the images. Some will be images the wrong way round where you look to be tapping your right cheek but one or more will be the correct way round and you will appear to be tapping your left cheek. This image is exactly the picture that other people see when they look at you.

Can you use a flat mirror to see round corners?
Yes, you can. Just set the mirror up at the corner you want to look round. Move away from the corner and look into the mirror. You should be able to see round the corner.

How does a periscope work?
Some people use a periscope when they are in a crowd. They use the periscope to see over the heads of other people. The periscope has two flat mirrors. They are arranged to see round corners. The mirror you look into is set at an angle so that when you look into it you look upwards. About half a metre above it is another mirror. This is set at an angle which lets you see a view in front of you but from about half a metre above your head.

Does a submarine telescope use mirrors?
No. It uses prisms. These are triangular blocks of glass. They can be used like mirrors to reflect light but they absorb much less light than a flat mirror and so produce brighter images. This is important in a submarine periscope which may be several metres long. If the image was not bright it could not be seen clearly at the other end of the tube.

Are there mirrors in a kaleidoscope?
Yes. There are three mirrors. They all face inwards and each one is set at an angle of 60° to the next one. There are objects in the kaleidoscope which form images in the mirrors. The objects and their images form patterns that you can see when you look into one end of the kaleidoscope.

Can you see an image of an image in a mirror?
Yes. If you hold up one mirror so that it shines into another mirror, you will see in each mirror many images of the other mirror. These images will be in a line and appear to disappear into the distance.

Why is the word 'ambulance' written the wrong way round on the front of an ambulance?
It is written this way so that when the ambulance comes up behind a car, the car driver can see the word the correct way round in the mirror and move over to let the ambulance pass. Some police cars also have the word 'police' written the wrong way round so that it can be read easily in a mirror.

Why does a mirror shine more than glass?
It has a thin layer of metal behind it. This forms a silvery coating, which reflects a large amount of light. Although you may see your reflection in a window it is not very bright because light from the other side of the window passes through the glass and weakens the image.

How does a two-way mirror work?
A two-way mirror may be used at a police station where a person is asked to identify someone from a line of people. A line of people stand on one side of a sheet of glass in a bright room. The person who is trying to make an identification stands on the other side of the glass in a dark room. As there is no light passing through the glass into the light room the people in the line-up see themselves in a mirror and do not see the person in the dark room. The light passes through the glass into the dark room allowing the person
to try and make an identification.