The phase difference between the light-waves emerging from the slits of Young’s experiment is $\mathrm{\pi }$ radian. Will the central fringe be bright or dark? Explain with reason.

State with reason whether diffraction of light takes place at the two slits in Young’s interference experiment.

Can two sodium vapour lamps (i.e., two independent monochromatic sources of light) act as coherent sources? Explain in brief.

Suppose the two slits in Young’s apparatus are illuminated by two identical but independent monochromatic light sources. Will you observe interference pattern on the screen?

The frequencies of two wave-sources $A$ and $B$ shown in the figure are equal. What type of interference will occur at the points $P$, $Q$ and $R$? The distance $AB=\raisebox{1ex}{${\displaystyle \lambda }$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ (assume that the waves from $A$ and $B$ start in the same phase).