# Wave Question

1. Jan 13, 2008

### Raza

1. The problem statement, all variables and given/known data:
Two coherent sources emit waves of the same frequency, amplitude and phase.

D1 and D2 are the distances of the sources S1 and S2 to point P respectively. How does the amplitude of the resultant wave at P compare with the amplitude of each individual wave when the absolute value of the differences D1 - D2 equals two and half wavelengths?

2. The attempt at a solution:

2. Jan 13, 2008

### Tom Mattson

Staff Emeritus
Your textbook must explain somewhere how path length difference is related to constructive and destructive interference. Can you find it, and try to apply it to this problem?

3. Jan 13, 2008

### Raza

I have no textbook but I keep looking in my notes and haven't found much help.
This is from a mid-term test.
This is the answer that I wrote in:
Because the wavelenght is an odd number, this means that the wave will be at it trough at that time. ????

The average mark on that test was 35%.

4. Jan 14, 2008

### Tom Mattson

Staff Emeritus
There's no textbook for the course you're taking??

You weren't told what the wavelength is, so how could you possibly know that it is an odd number? It doesn't matter anyway, because the answer doesn't depend on the wavelength. What matters is the path length difference. This is a major idea in physical optics, so you should become familiar with it. Basically, one of the ways two waves that are initially in phase can become out of phase is if they travel two paths of different lengths. If the path length difference is equal to an integer number of wavelengths, then the waves are in phase when they meet. If the path length difference is equal to a half-integer number of wavelengths, then the two waves are exactly (180 degrees) out of phase when they meet.

There is a brief explanation with illustrations at the following link:
http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html

Have a look at that, and let us know what you think.