Q.In a biprism experiment, light of wavelength 5200 Å is used to obtain an interference pattern on the screen. The fringewidth changes by 1·3 mm when the screen is moved towards the biprism by 50 cm. Calculate the distance between the two virtual images of the slit.

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). …