Mirrors come in a number of varieties:

• a plane mirror is flat and, if you look into it, you’ll get an exact mirror image of yourself
• a convex mirror bends outwards and your reflection will appear long and thing
• a concave mirror folds inwards and your reflection will appears short and fat

You see an image in a mirror due to the reflection of light. The normal is the line which is perpendicular to the mirror. When a ray of light is reflected there are two angles against it:

• the angle of incidence which is between the incident ray and normal
• the angle of reflection which is between the reflected ray and normal

Both of these angles are equal for rays reflected off a mirror.

The image reflected is the virtual image. The light rays which reflect off the mirror and into your eye seem to come from that image. However, unlike a real image it wouldn’t be possible to project a virtual image onto a screen. This is because a real image is created when light rays are focused onto a screen.

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Light refraction

When light rays change direction this is known as refraction. Refraction works in the following way:

• if the light ray travels along the normal then there’s no refraction
• if the light ray bends towards the normal, for instance if it travels from air into glass, then the angle of refraction (r) is smaller than the angle of incidence (i)
• if the light ray bends away from the normal, for instance when it travels from glass to air, then angle r is bigger than angle i

All types of wave can refract. Another example is water waves. Take a tank of water with a glass plate. On one side the water is deep and on the other side it’s shallow. As the water moves from one side to the other the waves change speed: the shallow water waves are slower. The change in speed means that they change direction too.

• As the waves move from the deep to the shallow end they slow down and move towards the normal.
• As the waves move from the shallow to the deep end they speed up and move away from the normal.

Similarly with light.

• As a light ray moves from air to glass it slows down and moves towards the normal.
• As a light ray moves from glass to air it speeds up and moves away from the normal.