# In the ideal YDSE when a glass plate (RI= 1.5) of thickness t is introduced in path of one of interfering beams , the intensity at the position where central maximum occurred previously remains unchanged. The minimum thickness of glass plate is :

$(a)\;\lambda \\ (b)\;3 \lambda \\ (c)\;4 \lambda \\ (d)\;2 \lambda$

Path difference due to slab should be integral multiple of $\lambda$ or $\Delta x = n \lambda$