Two coherent plane waves of identical frequency,polarization,and intensity $I$ interfere at a point where they differ in phase by $60^{\circ}$. What is the resulting intensity?

Consider the following statements about interference of light:
$A$. The interference fringes are equally bright and equally spaced.
$B$. At the centre of a bright fringe,the intensity is four times the intensity of the incident wave.
$C$. For constructive interference of two waves,the crest of one wave coincides with the trough of another wave.
Which of the above statements are correct?

In the phenomenon of interference:

Light waves are emitted by a common source. For what time interval will the phase remain constant?

Two coherent sources produce interference fringes with an intensity ratio of $81:1$. Find the ratio of the maximum to the minimum intensity in the interference pattern.

Light waves from two coherent sources arrive at two points on a screen with path difference of zero and $\lambda / 2$. The ratio of the intensities at the points is