What is interference of light? Write two essential conditions for sustained interference pattern to be produced on the screen.Draw a graph showing the variation of intensity versus the position on the screen in Young’s experiment when (a) both the slits are opened and (b) one of the slits is closed.

What is the effect on the interference pattern in Young’s double slit experiment when:

(i) Screen is moved closer to the plane of slits?
(ii) Separation between two slits is increased. Explain your answer in each case.

Interference

Interference is the phenomenon of waves where waves from two different coherent sources superimpose at a given point and creates fringes on a screen placed as some distance away.

variation of I with x

Conditions for interference

(a) the two sources of light must be coherent in nature and have same frequency.

(b) the amplitudes of the interfering light waves must be same.

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the fringe width is given as

b = D/d

so,

b ~ D

and 

b ~ 1/d

so,

with decrease in screen distance (D) the fringe width will decrease as well.

and

with increase in slit  separation (d) the fringe width will again decrease