Wave Optics Ques 2
2. Angular width of the central maxima in the Fraunhofer diffraction for $\lambda=6000$ $ \AA$ 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
[NEET Odisha 2019]
(a) $420 $ $\AA$
(b) $1800 $ $\AA$
(c) $4200 $ $\AA$
(d) $6000 $ $\AA$
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Answer:
Correct Answer: 2.(c)
Solution: (c) $ \text { As } \theta=\frac{2 \lambda}{a} $
$ \theta_0=\frac{2 \times 6000}{a} $ $\quad$ ……..(i)
$ \frac{\theta_1}{\theta_0}=\frac{\lambda_1}{6000}$ $\quad$ ……..(ii)
$ \Rightarrow \lambda_1=0.7 \times 6000\left(\text { as } \theta_1=0.7 \theta_0\right) $
$ \Rightarrow 4200 $ $\AA$
BETA
