Double slit - couple questions.

[Shinns]

1. Why are the slits necessary? Do 2 individual light sources of the same frequency without slits or cards create an interference pattern? If not, why not?

2. How do different frequencies of light alter the interference patterns? Any links to experiments of this nature?

3. How about 2 light sources of different frequencies?

 

[jmlaniel]

1. Why are the slits necessary? Do 2 individual light sources of the same frequency without slits or cards create an interference pattern? If not, why not?

2. How do different frequencies of light alter the interference patterns? Any links to experiments of this nature?

3. How about 2 light sources of different frequencies?

Here are some quick answers :

1. Yes, you can do that quite easily with two lasers (since the slits are there only to mimic two point-like light source). However, in order to get them to interfere, they must be coherent (this is not easy to do with two separate laser source since you need them to have exactly the same frequencies and to be coherent). This is why one can do the interference experiment with one laser, but by separating the beam in two beams. This is basically the Michelson interferometer.

2. Assuming that your two frequencies are coherent (same problem as in 1.), what you will see is the equivalent of frequency beating (as you get with sound wave slightly out of tune from each other) in the spatial domain (i.e. that you can see on the wall or a screen). If the two frequencies are close to each other, you will see a quick oscillation modulated by a frequency related to the frequency difference.

You can take a look at this link for the frequency beating :
http://en.wikipedia.org/wiki/Beat_%28acoustics%29"

3. This is quite difficult because you really need two coherent sources. Otherwise, the waves phase will be random and this will give an averaged pattern on the screen and you won't see any interference pattern.

Since coherence is really important to your questions, you can take a look at this link for further reading :
http://en.wikipedia.org/wiki/Coherence_%28physics%29"