Problem

In YDSE, when a glass plate of refractive index 1.5 and thickness

t

is placed in the path of one of the interfering beams (wavelength

\lambda

), intensity at the position where central maximum occurred previously remains unchanged. The minimum thickness of the glass plate is (1)

2\lambda

(2)

(2/3)\lambda

(3)

\lambda/3

(4)

\lambda