# Homework Help: Wave Interference ?

1. May 18, 2006

### pkc111

Could someone explain how so many waves can travel through the same place in the medium at the same time but not interfere with each other.

eg. ADSL, optic fibre telephone, mobile phones etc?

Thanks.

2. May 18, 2006

### Tide

Technically, the waves do interfere with each other. I think what you really mean to ask is "Why can they interfere with each other and then go merrily on their way after the interaction as though nothing had happened?"

3. May 18, 2006

### Alain12345

In phase wave pulses will add in amplitude when they pass eachother. But if two out of phase wave pulses pass eachother, decontructive interference will result. That is, their amplitudes will decrease.

4. May 18, 2006

### pkc111

Sort of.

To my way of thinking every receiver is at a point in the medium where interference is occuring between hundreds of waves there.
How come the signal comes out clear?

5. May 19, 2006

### Tide

Many waves can pass through a point but, unless there is at least partial absorption at that point, you cannot consider it a receiver.

The basic form of interference is the simple superposition of waves. That just means that the amplitude of the resultant wave is the sum of the propagating waves. The waves are otherwise uncoupled and retain their individual properties as though nothing has occurred. They will continue on unimpeded. This condition is characteristic of, say, electromagnetic waves propagating in vaccuum or, generally, "low amplitude" waves propagating in some medium. These waves are often referred to as "linear waves."

Another level of interference is when waves propagating in a medium have "high amplitude." This means that the waves either separately or in combination have sufficient intensity to modify the properties of the medium in which they are propagating. In such situations, the individual waves are coupled (via the medium) and their characteristics may indeed be altered. Their frequencies may change, they may be scattered off in other directions or they may be absorbed (either partially or completely) depending on the specfic circumstances. This situation is often called "nonlinear."

For all of the applications you mentioned in your first message the wave amplitudes are such that they fall squarely in the linear regime. Nonlinear effects are important in applications such as producing laser light or propagating very intense laser beams through the atmosphere.