 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 10 8 m/s  The speed of light is less in any other medium ◦ e.g., the speed of light in water is 2.26 x 10 8 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 10 8 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