Refraction D. Crowley, 2008

Refraction To know what refraction is, and why it happens

Light Look at the answers drawn by pupils to an exam, which asked the following question: a man looked into a mirror, and saw a lamp behind him Analyse the diagrams, and explain say which one you think is correct (and why the others are wrong – look carefully)!

Light Nita’s diagram is correct – ruler drawn line from lamp to mirror surface to the eye (at the correct angle) The rest are incorrect: - Brent’s diagram – ray line goes behind mirror Sara’s diagram – ray line was drawn freehand Sanjay’s diagram – ray line did not reach the mirror Niki’s diagram – ray line does not enter the eye

Refraction Light travels in straight lines Light can bend at the boundary between two materials with different densities - this is called refraction The light ray bends towards the normal as it enters The light ray bends away from the normal as it leaves The ray entering the block is parallel to the ray leaving the block, if the block has parallel faces A ray entering the block at 90° is not refracted

Speed air glass air = fastest glass = slower diamond = slowest The speed of light waves depends on the material they are travelling through If light waves enter a different material (e.g. travel from glass into air) the speed changes, causing the light to bend or refract glass air air = fastest glass = slower diamond = slowest

Experiment Using the ray box and glass blocks test for yourself the properties of refraction. As you carry out the experiment, complete the worksheet summary for your findings…

Experiment

Refraction

Refraction Why does the light ray bend towards the normal when it enters the glass block, and then bend away from normal as it leaves? In the muddy field it slows down as there is more friction If it enters the field at an angle then the front tyres hit the mud at different times First to hit the mud is tyre 1, and will move more slowly than tyre 2. This causes the car to turn towards the normal When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal Tyre 2 Tyre 1

Refraction If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down

Water Water is denser than air, so light is refracted when it travels through the surface of the water. This is why sticks seem to be bent if they are partly underwater, and why swimming pools look shallower than they really are Refraction lets you see objects that are normally hidden…

Archer Fish image of prey prey location The Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. on a leaf The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey image of prey The fish does not aim at the refracted image it sees, but at a location where it knows the prey to be prey location