Sustained interference patterns

[wakejosh]

Homework Statement

In order to produce a sustained interference pattern by light waves from multiple sources, which of the following conditions is sufficient?

a. the sources are coherent
b. the sources are monochromatic
c. no more than two sources are allowed
d. three or more sources are required
e. none of the choices above are valid

The Attempt at a Solution

This question seems messed up to me. From what I understand, both A and B can be correct. I am totally confused. please help if you can, thanks.

 

[wakejosh]

any help? my brain is melting.

 

[ananthu]

The sources should be coherent.

The coherent sources, by definition, should possesses same wave length, same amplitude and same phase or constant phase difference.

But a monochromatic source will possesses same wave length but not necessarily be of same phase or constant phase difference.