## Refraction

Think of a marching band that travels across a smooth concrete parking lot onto a rough and muddy grassy playing field. The band members can take larger steps and move faster on the concrete than on the grass. Yet, being band members, they all remain in step, taking steps at the same pace whether on concrete or grass. This causes the band to change its direction as it moves from the faster concrete to the slower grass.

Light behaves in much the same way as it moves from air, where it travels very fast, into another medium like glass or plastic or water, where it travel more slowly.

This bending of light is known as refraction. It is due to a difference in the speed of light in the two media. Instead of describing the speed of light directly, we usually talk about the index of refraction and label that as "n". The index of refraction is the ratio of the speed of light in vacuum, c, to the speed of light in a particular medium, v. That means

n = c / v

The index of refraction for water is

n = 1.33

That means the speed of light in water is

n = c / v = 1.33

v = c / 1.33

v = 0.75 c

Q: If light travels from water into glass, will it be bent toward or away from the normal?

A: The index of refraction for water is 1.33 and the index of refraction of glass is about 1.55 so the index of refraction of water is less than the index of refraction of glass. This is the same situation we had in the example oflight passing from air into water. So light will be bent toward the normal.

Q: What is the speed of light in a glass whose index of refraction is 1.60?

A: The index of refraction of a medium is

n = c/v.

We can multiply both sides by v and divide both sides by n to get

v = c/n

Now we can use numbers to calculate v, the speed of light in the glass,

v = ( 3.0 x 108 m/s ) / ( 1.6 )

v = 1.88 x 108 m/s

 Reflection Total Internal Reflection
Back to "Reflection and Refraction" Page