Interference

Construction and destruction
Whenever two or more beams of light (or another type of radiation) pass each other, interference occurs. But only in the space where both beams are.

 Interference in water
A discription of this effect is the example of the lake. When you throw a rock in a lake, you will see ripples coming from the spot where the rock fell into the water. These ripples can be compared with light, as they both have an amplitude, wavelength and direction. If you throw another rock in the lake, not far away from the first one, this one also "creates" ripples. The two waves will eventually meet each other, and where both of them are, interference occurs. At some places both wavetops or wavebottoms come together, at these places the waves will go twice as high, and twice as deep as when they were alone. At some other places the wavetop of one wave comes together with the wavebottom of the other wave. At these places the water will remain unmoved. The whole of this is called the interference pattern.

This effect is simulated in the following applet:

Beat frequency
In this example (and in the lake) every wave has the same amplitude and wavelength. If you vary the amplitude of one wave, the amplitude of the combined wave differs (i.e., at the place where both wavetops meet, the waves may go higher or lower). If however, the wavelength varies, both of them combine, and you see a wave that is a combination of the two waves. This effect is shown in the following applet. For the best result move one slider to one end, and the second to the other end. This effect also occurs when both waves have the same wavelength, but then they just amplify each other, as you have seen in the previous example. Frequencies of the resultant wave is equal to the difference of the individual frequencies. The wave with the lowest wavelength is shown in blue on the combined, this is just for making it a bit more easier to see the effect.

A radio, television or any other device which retrieve information from waves can filter out the right frequency it needs (imagine what the waves in the sky would look like with all those radio/television stations, it's just one big pool of interference) without getting disturbed by the others.

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