Two coherent sources of light can be obtained by

Radio waves coming at an angle $\alpha$ to the vertical are received by a radar after reflection from a nearby water surface and directly. What should be the height of the antenna from the water surface so that it records a maximum intensity? (wavelength $= \lambda$)

By a monochromatic wave,we mean

Two identical coherent sources are placed on a diameter of a circle of radius $R$ at a separation $x (x << R)$,symmetrically about the center of the circle. The sources emit waves of identical wavelength $\lambda$. Find the number of points on the circle with maximum intensity,given $x = 5 \lambda$.

Two point sources $X$ and $Y$ emit waves of the same frequency and speed,but $Y$ lags in phase behind $X$ by $2\pi l$ radians. If there is a maximum in direction $D$,the distance $XO$ (where $n$ is an integer) is given by:

Two waves having amplitudes in the ratio $5:1$ produce interference. The ratio of the maximum to minimum intensity is