Which optical phenomena can be explained by the wave nature of light?

Which of the following is the path difference for destructive interference?

Two sources are said to be coherent if they produce waves

Two coherent sources $S_1$ and $S_2$ and a screen are arranged as shown in the figure. If the distance between the two coherent sources is $n \lambda$ and the distance of the screen from the nearer coherent source $S_2$ is $D$,then the distance of the first bright fringe on the screen from the point $O$ is (where $\lambda$ is the wavelength of the light emitted by the coherent sources).

Two coherent sources of wavelength $\lambda$ produce a steady interference pattern. The path difference corresponding to the $10^{\text{th}}$ order maximum will be:

Two plane polarized light waves combine at a certain point whose electric field components are $E_1 = E_0 \sin(\omega t)$ and $E_2 = E_0 \sin(\omega t + \frac{\pi}{3})$. Find the amplitude of the resultant wave.