Three media are arranged in various ways as shown in figures a, b, c and d. Light of wavelength` lambda` is incident perpendicularly on the boundary of the middle layer and interference between waves reflected at the boundaries of the middle layer is studied. In which of the four cases the reflected light is eliminated by destructive interference when the thickness of middle layer approaches zero.` mu_(1) = 1.5" and" mu_(2) = 1.8`

Answer

a,b.