The phase difference between two light waves reaching a point is $\mathrm{\pi }/2$. What is the resultant amplitude in $\mathrm{mm}$ if the individual amplitudes are $3 \mathrm{mm}$ and $4 \mathrm{mm}$.

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.