# Reflection, Refraction and Diffraction

When a wave hits a barrier the wave will be reflected. If it hits the barrier at an angle then the angle of reflection will be equal to the angle of incidence. Echoes are caused by the reflection of sound waves.

When a wave moves from one medium into another, it will either speed  up or slow down. For example, a wave going along a rope will speed up if the rope becomes thinner. (This is why you can ‘crack’ a whip: the wave that is  sent down the whip accelerates until it breaks the sound barrier.) And sound going from cold air to hotter air will speed up. When a wave slows down, the wavefronts crowd together – the wavelength gets smaller.

When a wave speeds up, the wavefronts spread out – the wavelength gets larger. Note that in both cases, the same number of waves will pass you per second, the wavelength may have changed, but the frequency has not.

This surfer is successfully travelling along one wavefront.

If a wave enters a new medium at an angle then the wavefronts also change direction. This is known as refraction. The amount that the wave is bent by depends on the change in speed. Water waves are slower in shallower water than in deep water, so water waves will refract when the depth changes.

Wave fronts change shape when they pass the edge of an obstacle or go through a gap. This process is known as diffraction. Diffraction is strong when the width of the gap is similar in size to the wavelength of the waves.

In the first diagram the wavelength is much smaller than the aperture. An example is light coming in through a window and forming a beam of light across the room. In the second diagram, the wavelength is similar to the size of the aperture. Ocean waves spread out like this when they enter a harbour.

If the apertures are much smaller than the wavelength then the wave cannot go through the aperture at all, which is why the door of a microwave oven has an array of small holes for you to see inside. Light can travel in or out through the holes, but microwaves (with a wavelength of 12 cm) cannot do so.

Short-wavelength signals with a wavelength of less than 1 m are used by television and mobile phone transmitters. These wavelengths are much smaller than the size of buildings, so the waves do not bend round the buildings, and you will not get a good television signal or mobile phone reception if there is a building between you and the transmitter.

To reduce this problem for mobile phones, the mobile phone companies use many transmitters in a city, and switch your phone to the transmitter with the best path to the phone. Satellite television does not have a problem because the receiver on the house has a direct view of the satellite.

Long-wavelength radio signals, with a wavelength of 100 m or so, bend round buildings and hills, and can give a good reception anywhere in a city and even hundreds of kilometres from the transmitter.

Waves do not merely transfer energy from one place to another. They do interesting things in getting from here to there.

Most of the time, waves travel in a straight line, unless something changes. They come across a boundary, or a change in the medium, or a opening, or a corner. Then, they do other things.

When a wave hits a boundary, part or all of the wave is reflected. This means that it bounces back. this wave behavior is called reflection. The reflected wave is the inverse of the incident wave. By choosing carefully the location of the boundary, reflection is used to create standing waves. When you look at yourself in a mirror, you are using reflection.

Sometimes when a wave hits a boundary, part of the wave goes through into the other medium. When this happens, the wave either speeds up or slows down, depending on the characteristics of the second medium. This causes the wave to bend. This bending of the wave is called refraction. This is why things sound different underwater than they do in the air. This is why a spoon looks weird when it rests in a glass of water. This is why your glasses are able to change the direction of the light so that you can see.

Waves will also bend to go around a corner or through a slit. This property of waves is called diffraction. This is why you actually can hear your mom yell at you when she is in the other room.