# As Physics Unit 2

- Created by: harryli
- Created on: 02-10-12 21:15

First 359 words of the document:

AS Physics Unit 2

Waves and Quantum Behaviour

Superposition and Coherence

Waves are not physical objects therefore they do not bounce off each other when they

collide.

Instead, they combine temporarily and then continue to travel as before. This combination is

called superposition.

The interference caused by superposition can be constructive, where two peaks meet and

make a bigger peak, or destructive, where a peak meets a trough and they cancel out to

some extent.

The phase of a wave, how far it is displaced from 0 at that point, can be represented by a

phasor.

The resultant amplitude when two waves meet can be found by joining their phasors tip to

tip.

If the waves are at the same point in their cycle, they are said to be in phase, with a phase

difference of a multiple of 2 radians.

Waves in phase at a point in space interfere constructively. Their phasors point in the same

direction, giving the maximum possible resultant.

Waves at points in their cycle as far apart as possible are said to be in antiphase, with a

phase difference of an odd number times radians.

Waves in antiphase at a point in space interfere destructively. Their phasors point in

opposite directions, giving a minimum resultant.

Waves at different points interfere with each other to a lesser degree, whether this is

destructive or constructive can be worked out by drawing phasors.

Interference patterns can be established when waves line up so that the phase difference is

always the same this requires waves to be coherent. Sources are coherent when they have

the same wavelength and frequency and a fixed phase difference.

The phase difference of two waves depends on the difference in the lengths of the paths

between the sources and the detector, known as the path difference. If the path difference is

a whole number of wavelengths the phase difference is 0, if the path difference is a half

number of wavelengths the phase difference is /2.

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