Question

In Young's double slit experiment, if monochromatic light used is replaced by white light then :

• A. no fringes are observed
• B. only central fringe is white, all other fringes are coloured.
• C. all bright fringes become white.
• D. all bright fringes have colour between violet and red.

Answer 1

Answer: (B)

only central fringe is white, all other fringes are coloured.

The difference in path lengths, being zero at the center gives zero phase difference at the center for all wavelengths. Hence the center fringe will be white. But at other points on the screen, phase difference will be given by $\varphi =k\Delta x=\frac{2\pi }{\lambda }\Delta x$ where $\Delta x$ is the difference in path lengths. Since $\lambda$ is different for different colours, hence fringe width will be different for each colour and hence all other fringes will be coloured.