Question

Radius of $1^{st}$ dark ring of fraunhofers diffraction pattern produced by a circular aperture of radius 0.02 cm at focal plane of convex lens of focal length 20 cm and by light of wavelength $\lambda =600nm$ is

• A. 0.7 mm
• B. 0.36 mm
• C. 1.3 mm
• D. 2 cm

Answer 1

The correct option is B 0.36 mm

$sin\theta =\theta =\frac{1.22\lambda }{D}$ and $\theta =\frac{R_1}{f}$

$R_1=\frac{1.22\lambda f}{D}=\frac{1.22\times 600\times {10}^{-9}\times 20\times {10}^{-2}}{2\times 0.02\times {10}^{-2}}$

$R_1=0.36$ mm