Wave Properties

Straight waves directed at a certain angle to a hard flagt surface (the reflector) reflect off at the same angle.The angle between the reflected wavefront and the surface is the same as the anghle between the incident wavefront and the surface. Therefore the direction of the reflected wave is at the same angle to the reflector as the direction of the incident wave. This effect is observed when a light ray is directed at a plane mirror. The angle between the incident ray and the mirror is equal to the angle between the reflected ray and the mirror.

When waves pass across a boundary at which the wave speed changes, the wavelength also changes. If the wavefronts approach at an angle to the boundary, they change direction as well as changing speed. This effect is known as refraction.

Refraction can be shown by water waves in a ripple tank when they pass across a boundary from deep to shallow water at an angle to the boundary. Because they move more slowly in the shallow water , the wavelength is smaller in the shallow water and therefore they change direction.

Refraction of light is observed when a light ray is directed into a glass block at an angle (i.e. not along the normal). The light ray changes direction when it crosses the glass boundary. This happens because light waves travel slower in glass than in air.

Difraction occurs when waves spread out after passing through a gap or round an obstacle. The effect can be seen in a ripple tank when straight waves are directed at a gap.

• The narrower the gap, the more the waves spread out
• The longer the wavelength, the more the waves spread out

To explain why the waves are diffracted on passing through the gap, consider each point on a wavefront as a secondary emitter of wavelets. The wavelets from the points along a wavefront travel only in the direction in which the wave is travelling, not in the reverse direction, and they combine to form a new wavefront spreading beyond the gap

Satellite TV dishes in Europe need to point south, because the satellites orbit the Earth directly above the equator. The bigger the dish, the stronger the signal it can recieve, because more radio waves are reflected by the dish onto the aerial. But a bigger dish reflects the radio waves to a smaller focus, because it diffracts the wave less. The dish therefore needs to be aligned more carefully than a smaller dish, otherwise it will not focus the radio waves onto the aerial.

• When waves hit a barrier or boundary that they cannot pass through, they are reflected. When reflected, waves undergo a phase change of 180° or π radians.

• Phase Difference = (2πd)/λ.

• There are 2π radians in 360° or one full cycle.

Superposition is when two or more waves, overlap the resultant displacement at a point is equal to the sum of the individual displacements at that point. This is the principle of superpositions.

Displacement is a vector quantity, so the sign is important. At the point of overlap:

• If both waves have the same sign displacement (e.g both +ve or -ve) constructive interference (reinforcement) will occur.
• At the point of overlap, if one wave has a positive displacement and the other has a negative one, destructive interference (cancellation) will occur.

The waves have to be coherant:

• Same speed
• Constant phase difference
• Same frequency and wavelength