Refraction of Light by 2E2 Edna Chew

Refraction of Light
Refraction is the bending of a wave when it enters a medium where its speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media.
If you have ever half submerged a pen into water, you have probably noticed that the pen appears bent at the point it enters the water.
This optical effect is due to refraction.

When light travels through air it travels at 186,000 miles per second. As the light encounters a water surface some of the light is reflected and some of the light passes into the water. Upon entering the water, however, the light slows down by 46,000 miles per second. This change in speed causes the direction of the light ray to change. When we look at a pencil in a glass of water the light from the pencil in the water is being refracted, so it appears in a different location.

How refraction happens depends on the refractive index of the mediums and the angle between the light ray and the line perpendicular (normal) to the surface separating the two mediums (medium/medium interface).

The angle between the light ray and the normal as it leaves a medium is called the angle of incidence. The angle between the light ray and the normal as it enters a medium is called the angle of refraction.

When light passes from a substance of lower optical density to a substance of higher optical density the light is refracted at a smaller angle relative to the normal. (Figure 1)

When light passes from a substance of higher optical density to a substance of lower optical density the light is refracted a larger angle relative to the normal. (Figure 2)

Figure 1 Figure 2

Refraction is also responsible for rainbows and for the splitting of white light into a rainbow-spectrum as it passes through a glass prism. Glass has a higher refractive index than air and the different frequencies of light travel at different speeds (dispersion), causing them to be refracted at different angles, so that you can see them. The different frequencies correspond to different colors observed.
While refraction allows for beautiful phenomena such as rainbows, it may also produce peculiar optical phenomena, such as mirages and Fata Morgana. These are caused by the change of the refractive index of air with temperature.
Refractive Index
The refractive index of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium.
The refractive index n of a medium is defined as the ratio of the phase velocityc of a wave phenomenon such as light or sound in a reference medium to the phase velocity vp in the medium itself:

Acknowledgement:
http://en.wikipedia.org/
http://library.thinkquest.org/27066/lightrays/nlrefraction.html
http://www.ps.missouri.edu/rickspage/refract/refraction.html
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr.html
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/mch/refr/more.rxml