Question

A diffraction pattern is obtained using a beam of red light. What happens if the red light is replaced by blue light?

• A. No change
• B. Diffraction bands become narrower and crowded together.
• C. Band become broader and farther apart.
• D. Bands disappear altogether.

Answer 1

The correct option is B Diffraction bands become narrower and crowded together.

The distance between two successive maxima or minima, in a diffraction pattern, is given by,
$\beta =\frac{\lambda D}{a}$, where $a$ is the slit width.

From this equation, it is clear that,
$\beta \propto \lambda$

As ${\lambda }_{\text{blue}}<{\lambda }_{\text{red}}$, the diffraction bands will become narrower and, therefore, crowded.

Hence, $\left(B\right)$ is the correct answer.

Why this question? To understand, how $\beta$ and the diffraction pattern changes, as $\lambda$ is changed.