Figure shows two identical narrow slits and .
A very small completely absorbing strip is placed at distance '' from the point .
'' is the point on the screen equidistant from and .
Assume () where and have usual meaning.
When is covered, the force due to light acting on strip is '' and when both slits are opened the force acting on strip is . Minimum positive '' () coordinate of the strip in terms of and is .
What is the value of ?