Question

Two monochromatic coherent point sources $S_1$ and $S_2$ are separated by a distance $L$. Each source emits light of wavelength $\lambda$, where $L>>\lambda$. The line $S_1{S}_2$ when extended meets a screen perpendicular to it at a point $A$.

• A. The interference fringes on the screen are circular in shape
• B. The interference fringes on the screen are straight lines perpendicular to the line $S_1{S}_{2}A$
• C. Point $A$ is an intensity maxima if $L=n\lambda$
• D. Point $A$ is always an intensity maxima for any separation $L$

Answer 1

The correct option is A The interference fringes on the screen are circular in shape

This is a setup of fabry-perot interferometer. Due to axial symmetry around $S_1{S}_2$ line, the path difference along the plane containing A has circular symmetry. Thus the interference fringes form circular rings.

If point A is a maxima, path difference between the two rays is $n\lambda$=Distance between the two sources=$L$.