Light travels through space at a speed of 3x108 ms-1 and in straight lines (ignoring relativity theory, etc!)
When light hits a material, three possible things can happen:
- Light can bounce off the surface of the material (reflection).
- Light can be transmitted through the material.
- Light can be absorbed by the material it's travelling through.
X-rays are a good example of this, as they are absorbed by bone but can be transmitted through flesh.
When light passes into another transparent medium (air, glass, perspex) it slows down. The more optically dense the material, the more the light slows.
If the light hits the new material at an angle it changes direction. We say that refraction has taken place.
In this example, the ray of light has been bent towards the normal.
But why does it bend?
Imagine a car travelling down a road. If the wheels on the right suddenly hit quick sand that side of the car would try to slow down, which would pull the car around to the right. It's the same for light. On the diagram above the top of the ray of light hits the glass boundary first and slows down. This has the effect of pulling the ray of light around in a new direction.
As the light emerges from the glass it speeds up again and bends away from the normal line. The emergent angle is equal to the angle of incidence.
Any part of the electromagnetic spectrum has a frequency that decides what type of wave it is. This frequency does not change when the wave is refracted. If the speed of the wave is reduced the wavelength of the wave must therefore also be reduced as:
Speed = frequency x wavelength
So the wavelength of blue light in air will be slightly longer than the wavelength of blue light in glass!
Note: Sound waves can also be refracted, but they travel faster in solids than in air, so everything is back to front!