Q.In a biprism experiment, the fringes are observed in the focal plane of the eye-piece at a distance of 1.2 m from the slit. The distance between the central bright band and the 20th bright band is 0.4 cm. When a convex lens is placed between the biprism and the eye-piece, 90 cm from the eye-piece, the distance between the two virtual magnified images is found to be 0.9 cm. Determine the wavelength of light used.

When two waves of the same frequency overlap, the superposition principle says their resultant displacement at any point is simply the sum of what each wave would produce alone. Where crests coincide with crests (or troughs with troughs), the waves reinforce (constructive interference, maximum intensity); where a crest meets a trough, they cancel (destructive interference, minimum/zero intensity). Since intensity depends on amplitude SQUARED, this produces a stable, spatially non-uniform pattern of bright and dark regions -- interference -- first demonstrated for light by Thomas Young in 1801, providing the first direct proof of light's wave nature (interference has no explanation under a purely particle picture of light). …