NEET-XII-Physics
17: Light Waves
Qstn# iv-41 Prvs-Qstn
- #41A convex lens of diameter 8.0 cm is used to focus a parallel beam of light of wavelength 620 nm. If the light is focused at a distance of 20 cm from the lens, what would be the radius of the central bright spot formed?Ans : Given:
Wavelength of the light used, `` \,\mathrm{\,\lambda \,}=620\,\mathrm{\,nm\,}=620\times {10}^{-9}\,\mathrm{\,m\,}``
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Diameter of the convex lens, `` d=8\,\mathrm{\,cm\,}=8\times {10}^{-2}\,\mathrm{\,m\,}``
Distance from the lens where light is to be focused, `` D=20\,\mathrm{\,cm\,}=20\times {10}^{-2}\,\mathrm{\,m\,}``
The radius of the central bright spot is given by
`` R=1.22\frac{\lambda D}{d}``
`` =1.22\times \frac{620\times {10}^{-9}\times 20\times {10}^{-2}}{8\times {10}^{-2}}``
`` =1891\times {10}^{-9}\,\mathrm{\,m\,}\approx 1.9\times {10}^{-6}\,\mathrm{\,m\,}``
∴ Diameter of the central bright spot, 2R = 3.8 × 10-6 m.