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

How do source and images behave as coherent sources?

What is the importance of coherent sources in case of interference of light ? How are coherent sources produced ?