In the ideal double-slit experiment, when a glass-plate (refractive index 1.5) of thickness t is introduced in the path of one of the interfering beams (wave-length λ), the intensity at the position where the central maximum occurred previously remains unchanged. The minimum thickness of the glass-plate is

$2λ$ $\frac{2λ}{3}$ $\frac{λ}{3}$ $λ$

$2λ$

Path difference = $(μ - 1)t = nλ$; For minimum t, n = 1; $∴ t = 2λ$