Question

Angular width of the central maxima in the Fraunhofer diffraction for $\lambda$ = 6000 � is $\theta_{0}$. When the same slit is illuminated by another monochromatic light, the angular width decreases by 30%. The wavelength of this light is,

• A. 1800 �
• B. 4200 �
• C. 6000 �
• D. 420 �

Answer 1

The Correct Option is B

Angular width $\propto \frac{\lambda}{d}$
$\Rightarrow \frac{\theta_{0}}{0.7\theta_{0}} = \frac{\frac{6000\,�}{d}}{\frac{\lambda}{d}} $
$ \Rightarrow \lambda = 4200 \,�$