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Diagram of refraction
When viewing at a certain angle, the straw appears to bend, due to refraction of light as it moves into the air

Refraction is the change in direction of a wave, caused by the change in the wave's speed. Examples of waves include sound waves and light waves. Refraction is seen most often when a wave passes from one medium to a different medium. Different types of medium include air and water.

When a wave passes from one medium to another medium, the wave will change its speed and its direction. For example, when a light wave travels through air and then passes through water, the wave will slow and change direction.

As light travels through the refractive device, it will 'bend' toward the normal.[1] When it leaves the refractive device to air (with a lower refractive index) it will return along the same angle as when it entered the device.

An example of how refraction works is placing a straw in a cup of water, with part of the straw in the water. When looking at a certain angle, the straw appears to bend at the water's surface. This is because of the bending of light rays as they move between the air and the water.

A good and simple way to understand how light works is to think of it like a car. When the car hits the gravel surface (this is the medium) on an angle, the tire that hits it first will slow, causing it to turn in that direction. Therefore, if light hits a medium on the right having a greater refractive index, it will bend right. The amount of bending is given by Snell's law.

When light refracts in a prism, it splits into colours of the rainbow because some wavelengths bend more than others.


  1. 'Normal' in geometry means vertical (perpendicular) to a surface or object.