Q. 274.5( 2 Votes )

Two coherent point sources S1 and S2 vibrating in phase emit light of wavelength λ. The separation between the sources is 2λ. Consider a line passing through S2 and perpendicular to the line S1S2. What is the smallest distance from S2 where a minimum of intensity occurs?

Answer :

The following diagram describes the situation.


Here, forms a right angled triangle.


The path difference is {from Pythagoras theorem}


To have a minimum at O, the path difference should be .


Therefore,



Squaring both sides,





Now, for n=0,



for n=1,



for n=2,



As negative values are not allowed, the minimum values is .


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