Two coherent plane waves of identical frequency, polarization, and intensity $I$ interfere at a point where they differ in phase by $60^{\circ}$. What is the resulting intensity?

Light exhibits wave nature because...

$PQ$ represents a wavefront and $AO$ and $BP$ are the corresponding two rays. Find the condition on $Q$ for constructive interference at $P$ between ray $BP$ and the reflected ray $OP$.

Two independent monochromatic light sources produce waves given by $y_1 = 2 \sin \omega t$ and $y_2 = 3 \cos \omega t$. Which of the following statements is true?

The following figure shows sources $S_1$ and $S_2$ that emit light of wavelength $\lambda$ in all directions. The sources are exactly in phase and are separated by a distance equal to $1.5\lambda$. If we start at the indicated start point and travel along path $1$ and $2$,the interference produces a maxima all along:

$A$ beam with wavelength $\lambda$ falls on a stack of partially reflecting planes with separation $d$. The angle $\theta$ that the beam should make with the planes so that the beams reflected from successive planes may interfere constructively is (where $n = 1, 2, \dots$)