What are coherent sources of light?

Consider the following statements in case of Young’s double slit experiment. (a) A slit S is necessary if we use an ordinary extended source of light (b) A slit S is not needed if we use an ordinary but well collimated beam of light (c) A slit S is not needed if we use an spatially coherent source of light. Which of the above statements are correct ?

In the biprism interference experiment, what is the nature of the source of light.

Light from two coherrent sources of same amplitude (A) and wavelength (lambda) illuminates the screen. The intensity of central maximum is I_0 . If the source were incoherent, the intensity at the same point will be :

The light waves from two coherrent sources of same intensity I interfere. At the minimum, the intensity of light is zero. What is the intensity of light at the maximum ?

In Young’s experiment, the ratio of maximum to minimum intensities of the fringe system is 4 : 1 . The amplitudes of the coherent sources are in the ratio

What is the commercial sources of helium?

Interference is produced with two coherrent sources of same intensity. If one of the source is covered with a thin film so as to reduce the intensity of light coming out of it to half,