When is the interference constructive?

When is the interference destructive?

What is interference of light?

What happens to the interference pattern,when one of the slits is closed?

In Young's double-slit experiment deduce the condition for constructive and destructive interference at a point on the screen. Draw the graph showing variation of the resultant intensity in the interference pattern against position 'X' on the screen.

In which case no interference is produced?