Question

Assertion : The fringe closest on either side of the central white fringe in case of interference pattern due to white light is red and the farthest appears blue.
Reason : The interference patterns due to different component colours of white light overlap.

• A. If both assertion and reason are true and reason is the correct explanation of assertion
• B. If both assertion and reason are true but reason is not the correct explanation of assertion
• C. If assertion is true but reason is false
• D. If both assertion and reason are false

Answer 1

Answer: (C)

The interference patterns due to different component colours of white light overlap incoherently.

The central bright fringes for different colours are at the same position. Therefore, the central fringe is white.
For a point $P$ for which
$S_2 P - S_1 P = \frac{\lambda_b}{2}$ where $\lambda_b \approx 4000 \mathring{A}$
represents the wavelength for the blue colour, the blue component will be absent and the fringe will appear red in colour. Slightly farther away where
$S_2Q — S_1Q = \lambda_b = \frac{\lambda_r}{2}$
where $\lambda_r \approx 8000 \mathring{A}$ is the wavelength for the red colour, the fringe will be predominantly blue. Thus the fringe closest on either side of the central white fringe is red and the farthest will appear blue.