Refraction

Any time the speed of a wave is changed, the direction of the waves is bent; this characteristic of waves is called refraction.

When a wave is slowed down, (by entering a different medium, or by encountering friction) it is refracted in such a way that its wave front bends towards the normal.

When a wave enters a medium in which it speeds up, it is refracted in such a way that it bends away from the normal.

If light enters a new medium along the normal, it doesn’t bend at all, no matter how much it speeds up or slows down

Examples of the refraction of light:

A mirage occurs when light that would normally have hit the ground far in front of you, encounters a thin layer of hot air near the ground (in which it travels faster), bends away from the normal, and hits your eye.

What you are actually seeing in a mirage is the sky.

A fish in the water looks like it is in a different location because its image is refracted, but your brain thinks that light has gone straight.

A lens uses refraction to change the direction of light.

If a lens causes the light rays to converge, then can focus light and it can be used to magnify.

Since light travels at a different speed through every different substance, it is refracted more by some substances than by others. Each transparent substance has an Index of Refraction, or number that tells how much it refracts light. The higher the index of refraction, the more light is slowed, and the more it is refracted.

c

The index of refraction is a number without units. It is found by using this equation: n = v

Where “n” is the index of refraction, “c” is the speed of light in a vacuum (3 ´ 10⁸ m/s) , and “v” is the speed of light in the medium (water, glass, etc)

Example: light travels through water at 2.3 ´ 10⁸ m/s, what is the index of refraction?

N = 3.0 ´ 10⁸ m/s ¸ 2.3 ´ 10⁸ m/s = 1.3 (notice that the units cancel out completely)

You could also use the equation n = c / v to find the velocity of light in given medium if you know “n”

Index of Refraction for Various Substances

Diamond 2.4 Cubic Zirconium 2.20

Fluorite 1.43 Benzene 1.50

Quartz 1.45 Glycerine 1.47

Glass 1.52 Water 1.33

Snell’s Law: n_i (sin q_i) = n_r (sin q_r)

where n_i is the index of refraction of the first substance and q_i is the angle of the incidence, and n_r is the index of refraction of the second substance and q_r is the angle of refraction.

What is the index of refraction of the unknown substance?

n_i (sin q_i) = n_r (sin q_r)

1.33 (sin 70^o) = n_r (sin 45^o)

1.249/0.707 = 1.766 = n_r