Refraction is a property of light in which the speed of light and its direction of travel change as it passes from one medium to another

Air

Water

Light travels in a straight line and at a constant speed as long as the medium it is travelling in remains the same

When light travels from one medium to another, however (e.g., from air to water), the light rays bend, or refract◦ This means that both its direction and speed

changes

Light travels as a wave Crests and troughs of waves can be thought

of as wave fronts, with rays perpendicular to them showing the direction the wave is travelling in

Think of a marching band:

The mud is sticky, so students cannot march as fast in it As each student reaches the mud, he or she slows down The slower students “pull” the line back and cause a bend

in the line, representing the wave front As a result, the direction in which the entire row is

marching changes – light behaves the same way

The exact path of light as it travels from one medium to another can be found by applying Fermat’s Principle:

When light travels from one point to another, it follows the path that will take the least time.

In a single medium, the path that takes the least time is a straight line

When travelling from one medium to another, the path that takes the least time is not a straight line

Compare the dashed line with the solid, bent line going from point A in the air to point B in the water

In air, where light travels faster, the solid line is longer than the dashed line

In water, where light travels slower, the solid line is shorter than the dashed line

Light travels a longer distance in air and a shorter distance in water than it would if it followed a straight line

Following the bent path (solid line) takes less time than following the straight path (dashed line).

Mostly similar terminology to reflection The refracted ray is the ray that is bent upon

entering a second medium The angle of refraction, R, is the angle

between the normal and a refracted ray

If a light ray travels from a medium in which its speed is faster (such as air) to a medium in which its speed is slower (such as water), the refracted ray bends towards the normal

If a light ray travels from a medium in which its speed is slower to a medium in which its speed is faster, the refracted ray bends away from the normal

In summary:

◦Faster medium Slower medium = Bends towards the normal

◦Slower medium Faster medium = Bends away from the normal

How much a light ray refracts is determined by how much the speed of light changes as it travels from one medium to another

In a vacuum (such as space), the speed of light is 3.00 x 108 m/s

The speed of light is less in any other medium◦ e.g., the speed of light in water is 2.26 x 108 m/s

It is inconvenient to use these large numbers to describe relative speeds, so we use the index of refraction instead

The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in a given medium

Example: calculate the index of refraction of water if light travels through it at 2.26 x 108 m/s

(Found on pg. 454)

Dispersion is the process of separating colours by refraction

Each colour of light travels at a slightly different speed in any medium except a vacuum (where they all travel the same speed)

Can see this by shining white light into a prism

White light contains all the wavelengths (i.e., colours) of visible light

When white light is shined into a prism, it refracts twice – once when it enters, and once when it leaves

As it exits, the light is separated into a spectrum of colours

Blue light bends more than red light, so it must be travelling at a slower speed