Two light waves superposing at the midpoint of the screen are coming from coherent sources of light of phase difference $3\mathrm{\pi } \mathrm{radian}$. Their amplitudes are $1\mathrm{cm}$ each. What will be the resultant at the given point.

In Young's Double slit experiment, what is the intensity at a point on screen where the two waves arrive having a phase difference of 

$\left(i\right) 60°\left(ii\right) 90°and \left(iii\right) 120°$. Assume that the intensities of each source is $I_0$.

Find the ratio of intensities of two points $P and Q$ on a screen in Young's double-slit experiment when waves from sources $S_1 and S_2$ have phase difference of $\left(i\right)\pi /3 and \left(ii\right)\pi /2$ respectively.