Friday, March 6, 2009

Reflection and Refraction


Waves (and in particular light) is reflected off of surfaces. If we take the example of rays of light on a mirror, a virtual image is formed. This is upright, the same size, and appears to be directly behind the mirror.

If a ray of light is beamed at a mirror, this will be reflected. The normal to the block is perpendicular to it, the angles are measured in reference to this. The ray going in is called the incident ray and the acute (smaller) angle it makes with the normal is called the angle of incidence. See diagram below to understand this.

The angle the reflected ray leaves is equal to the angle of incidence. I.e. angle of incidence = angle of reflection.


Refraction has light waves changing direction. This occurs as a result of the light changing speed when it goes from one medium to another. The speed of light varies according to what it is travelling through, the standard given is the speed in a vacuum, this is 299 792 Km/s, the speed of light in water is 225 000 Km/s.

Take a look at the example of refraction shown below. The light ray is travelling through tank of water.

When the light enters the water, it slows down so moves towards the normal. When the light emerges from the tank into air, it speeds up so it moves away from the normal.

Refraction in water is the reason that when you stand beside a swimming pool and look down at it, it appears to be shallower than it really is. Also, if you put a straw at an angle in a cup of water (or juice or whatever), it will appear to bend at the water-air boundary.

Total Internal Reflection (TIR)

The inner surface of a glass block will begin acting like a mirror if a ray of light strikes it at the critical angle, this is 42°. This value is less in other mediums, for instance it is 49° between water and air.

When the angle of incidence is equal to the critical angle, the light will emerge along the edge of the block, but when the angle of incidence is more than the critical angle, it will be reflected. The diagram below illustrates this.

This phenomenon is put into practical use in fibre optic cables. Where light signals will travel along a glass fibre providing the angle of incidence exceeds the critical angle of 42°. Fibre optic cables are used to send telecommunication information such as the internet.

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