CBSE Class 12 Physics 2017 Outside Delhi Set 1 Paper

Question : 15 of 26

Marks: +1, -0

(a) Monochromatic light of wavelength

589 {\nm}

is incident from air on a water surface. If

µ

for water is 1.33 , find the wavelength, frequency and speed of the refracted light.
(b) A double convex lens is made of a glass of refractive index 1.55 , with both faces of the same radius of curvature. Find the radius of curvature required, if the focal length is

20 {\cm}

Solution:

(a) Calculation of wavelength, frequency and speed
(b) Lens Maker's Formula
Calculation of

R

(a)

λ=\;{589 {\nm}}/{1.33}=442.8 {\nm}

Frequency

v=\;{3 × 10^8 {\ms}^{-1}}/{589 {\nm}}

=5.09 × 10^{12} {\Hz}

Speed

v=\;{3 × 10^8}/{1.33} {\m} \/ {\ s}

=2.25 × 10^8 {\m} \/ {\ s}

\;\text " (b) "\; \;{1}/{f}\;= [\;{µ_2}/{µ_1}-1] [\;{1}/{R_1}-\;{1}/{R_2}]

∴ \;\;{1}/{20}= [\;{1.55}/{1}-1] \;{2}/{R}

∴ R\;=(20 × 1.10) {\cm}=22 {\cm}

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