Which of the following is conserved when light waves interfere?

$A$ ray of light from a monochromatic point source of light is incident at a point on the screen. If a thin mica film of thickness $t$ and refractive index $n$ is introduced in its path,then the optical path

Two waves of intensity $I$ undergo interference. The maximum intensity obtained is

When a light wave enters from air into water, which quantity does not change?

Two coherent sources whose intensity ratio is $64: 1$ produce interference fringes. The ratio of intensities of maxima and minima is

Two coherent sources of intensities $I_1$ and $I_2$ produce an interference pattern on a screen. The maximum intensity $I_{max}$ in this interference pattern is: