Two coherent sources of light can be obtained by

To observe a stationary interference pattern formed by two light waves,it is not necessary that they must have :

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?

Two identical light waves having phase difference $\phi$ propagate in the same direction. When they superpose,the intensity of the resultant wave is proportional to:

Interference fringes are produced on a screen by using two light sources of intensities $I$ and $9I$. The phase difference between the beams is $\frac{\pi}{2}$ at the point $P$ and $\pi$ at the point $Q$ on the screen. The difference between the resultant intensities at point $P$ and $Q$ is (in $I$)

Write down the principle of superposition and what is interference? Write down its types.