Question: 21. If the monochromatic source in Young’s double slit experiment is replaced by white light, then
(1) interference pattern will disappear.
(2) there will be a central dark fringe surrounded by a few coloured fringes.
(3) there will be a central bright white fringe surrounded by a few coloured fringes.
(4) all bright fringes will be of equal width.
Answer: Option (3)
Explanation:
White light consists of a continuous range of wavelengths.
At the central point on the screen, the path difference between light coming from the two slits is zero.
For zero path difference, constructive interference occurs for all wavelengths present in white light.
As a result, all colours overlap at the center, producing a bright white fringe.
On either side of the central fringe, the path difference is non-zero.
Since different wavelengths satisfy the condition for constructive interference at different positions
given by \beta = \frac{\lambda D}{d}, the fringes for different colours do not coincide.
This leads to the formation of a few coloured fringes on both sides of the central white fringe.
Beyond a certain distance, the coloured fringes overlap and become indistinct.
Hence, a central bright white fringe surrounded by a few coloured fringes is observed.