A lot of questions about wave optics

[haha1234]

Homework Statement

In this picture,the ray b and c are parallel.So why there are interference occur?

Homework Equations

The Attempt at a Solution

 

[BOYLANATOR]

You can't think of it as one single ray of light. You must think of lots of parallel lights incident on the surface. Can you see how interference will occur then?

 

[haha1234]

You can't think of it as one single ray of light. You must think of lots of parallel lights incident on the surface. Can you see how interference will occur then?

Sorry,I don't understand. Although there is a lot of rays,they are parallel thus they cannot combine.So I cannot imagine how interference can occur.

 

[ehild]

The light waves are extended, similarly to water waves in the picture: crests and troughs traveling in a certain direction. They can be imagined of infinite extension, - infinite with respect to the wavelength. A "ray" represents a normal of such "wavefronts", showing the direction of travel of the wave. You can draw a ray everywhere.

The wave reflected from the first surface of the layer is also a wave with its crests and troughs, and there is an other wave reflected from the back surface of the layer. There is some electric field corresponding to both reflected ways. These fields combine (interfere) making the resultant reflected wave. If the crests of the first reflected wave coincide with crests of the second wave, they intensify each other, (constructive interference). When the crest of the first reflected wave coincide with troughs of the second one, the resultant wave is weak, it is destructive interference.

ehild