What are coherent sources?

(Attempt any nine of the following questions) What are coherent sources? In a Young's double slit experiment the intensity of light at a point on the screen where path difference lambda is K units. Find the intensity at a point where the path difference is lambda/ 3 .

What are coherent source of light? Why no interference pattern is observed when two coherent source are too close

What is interference of light? Write two essential conditions for sustained interference pattern to be produced on the screen. Why interference pattern is not detected when two coherent sources are far apart?

Find the expression of fringe-width beta= (lambdaD)/d for Young's double slit interference pattern, where d is the separation between the two coherent sources.

Can non-coherent sources produce interference?