Question

If two independent sodium vapour lamps are used as a source in Young's double slit experiment. Then,
(A) Interference pattern is disappeared
(B) Only dark fringes are formed
(C) Interference pattern becomes more sharp
(D) None of these

Answer 1

In the Young’s double slit experiment, the same monochromatic light source is used in the two adjacent slits to observe the interference pattern. Monochromatic waves are those which produce a single wavelength.

Complete solution: When two independent sodium vapour lamps are used in Young’s double slit experiment, the interference pattern disappears as the phase difference would not be constant. Only a uniform illumination can be seen on the screen.

The correct option is A.

Note: The interference of the light shows the wave nature of the light. For the interference to happen, the two light waves should be correlated and are dependent on the phase difference between them. This condition would be met only if the two light waves are emitted from the same light sources so that they are dependent. This ensures the coherence between them which is one of the main criteria for the interference to be observed and this is not possible if the two light sources are independent. The meaning of the coherence of the light is how well they are correlated. Two independent sources of light will have a random phase difference between the light waves, which also changes continuously and hence the interference patterns cannot be observed for this case. Another criterion for the interference is that the light sources should be monochromatic.