Question

In Young's double slit experiment, if the two slits are illuminated with separate sources, no interference pattern is observed because

• A. There will be no constant phase difference between the two waves
• B. The wavelengths are not equal
• C. The amplitudes are not equal
• D. None of the above

Answer 1

The correct option is A

There will be no constant phase difference between the two waves

In conventional light source, light comes from a large number of independent atoms, each atom emitting light for about 10^-9 sec i.e. light emitted by an atom is essentially a pulse lasting for only 10^-9 sec. Light coming out from two slits will have a fixed phase relationship only for 10^-9 sec. Hence any interference pattern formed on the screen would last only for 10^-9 sec, and then the pattern will change. The human eye can notice intensity changes which last at least for a tenth of a second and hence we will not be able to see any interference pattern. Instead due to rapid changes in the pattern, we will only observe a uniform intensity over the screen.