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 of wavelength λ, the intensity at the position where the central maximum occurred previously remains unchanged. The minimum thickness of the glass plate is:
(1) 2λ
(2) \frac{2}{3}\lambda
(3) \frac{\lambda}{3}
(4) λ