Curved mirrors
Have you been in a hall of mirrors? This is a sideshow at a fairground or you may find one in a science museum. When you go in, you find a number of mirrors. When you stand in front of each one in turn you see an amazing image of yourself. The mirror may transform you into a wide-headed monster with a long neck, a barrel-shaped being with short legs or a creature with a long thin head but a big round body. The power to transform you is simply due to the shape of the mirrors.

Most of the mirrors that you will ever use are flat mirrors. When a light ray strikes the surface of a flat mirror, the reflected ray leaves in the opposite direction but at the same angle as the striking ray approached the mirror. This produces a reflection or image in the mirror, which has the same dimensions as the object from which the light came. For example, if you looked in a flat mirror you would see an image of your face that appeared to be the same distance behind the mirror as you are in front of it. You would find that the image of your face was the opposite way round to how it actually appears but it was not distorted. This means that when you scratched your nose with your left hand your image scratched its nose with its right hand but the size and shape of your nose was the same in the image as it was on your face. When you look in a mirror with a curved surface the image that is produced is quite different.

When a ray of light strikes a curved surface, the reflected ray leaves the mirror at a different angle from the angle of the approaching ray. This causes the image to be distorted.

A mirror can be curved in one of two ways. It can be curved inwards. Mirrors which are curved this way are called concave mirrors. You may think of them as forming a dish. A mirror can also be curved outwards. A mirror which bulges outwards is called a convex mirror.

You can investigate a concave mirror and a convex mirror very simply with a large shiny spoon and a pencil. Turn the spoon so that the concave surface is towards you. You should see an image of yourself in the spoon. The image will appear to be in the distance and upside down. Now bring the pencil point towards the centre of the spoon. At first it will appear upside down and become enlarged as it is brought nearer the spoon. At a certain point the image will flip over and become the right way up but it will still appear to be magnified.

When you turn over the spoon so that the convex underside faces you, the first thing you will notice is the field of view. It suddenly appears more distant and much wider than the field of view in the concave side of the spoon. When you hold the pencil close to you then push it nearer to the spoon, you see the pencil is always the same way up - the correct way up. As the pencil comes closer to the spoon you can see that the image is not magnified but actually appears smaller than it really is.

In a hall of mirrors each mirror you look into may have a combination of concave and convex surfaces to make your image bend into amusing shapes. If you get a chance to have a look in a hall of mirrors take it and work out how the surfaces of the mirrors are bending your face and body into monstrous proportions.

Do concave mirrors have any use?
Yes, they do. A concave mirror is used for making a shaving mirror or a make-up mirror. When a person is using a shaving mirror the face is brought close to the mirror so that an enlarged image of the skin can be seen. This helps the person to see if all the bristles in the skin have been cut down to the same length. When a person is using a make-up mirror, an enlarged image of the skin can also be seen. This time the image allows the person to see if there are any patches on the skin which have not been covered with make-up.

Are there any more uses for concave mirrors?
Yes. They are used in car headlights. They are not used to make reflections but to send out a strong beam of light rays. To understand how this works you could try the following activity. Put your fingers over the front of a torch so that a slit of light shines through. Point the light across a sheet of white paper. Put a spoon with its concave side towards the light so that half of it is above the paper and half below. When you do this you will see light rays being directed from the surface of the spoon to a few centimetres along the paper. The light rays come together to make a bright spot of light. When a concave mirror is used in car headlights, a bulb is placed at the spot where light would come together. This has the effect of making the opposite happen. The rays of light shining out from the bulb are reflected off the concave mirror to make a strong beam of light, which travels out in front of the car to shine on the road.

Are concave mirrors used to catch light?
Yes, they are. You may see a concave mirror near the base of a microscope. The mirror has a special mounting so that it can be turned in any direction. The purpose of the mirror is to collect light from a lamp and shine it up onto the specimen on the microscope. The mirror must NEVER be pointed towards the Sun to collect light as this could blind a person who then looked down the microscope.

Some telescopes look like wide drainpipes. These telescopes have a concave mirror at one end. The mirror collects light from distant stars and shines it onto a flat mirror. An eyepiece, which contains lenses, is set into the side of the telescope. When a person looks down the eyepiece a view of stars is seen that cannot be seen with the naked eye.

Do convex mirrors have any use?
Yes, they do. A convex mirror gives a very wide field of view. This makes them particularly useful as security mirrors in stores. A security guard can look up into a security mirror and watch a large number of people in the store. He or she can see if a customer is about to steal anything. Convex mirrors are also used in cars and trucks. They let the driver have a wide field of view of the cars behind. This is particularly useful on motorways where there may be three or four lines of vehicles approaching from behind. A convex mirror may also be used where a driveway with hedges or walls on both sides enters a road. The mirror is placed at the end of the driveway so that a driver there can see the traffic on the road and only enter the road when it is clear there is a gap.