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NCERT Class XII Chapter
Ray Optics and Optical Instruments
Questions With Solutions

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Question : 7
Total: 38
Double-convex lenses are to be manufactured from a glass of refractive index 1.55, with both faces of the same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm?
Solution:  
Both faces should be of same radius of curvature
|R1| = |R2| = R
So, lens makers formula
1
f
 = (
a
‌
µg−1) [
1
R1
−
1
R2
] ⇒
1
f
 = (1.55 - 1) [
1
R
−
1
−R
]
1
f
 = 0.55 [
2
R
] ⇒
1
20
 =
1.10
R
 ⇒ R = 20 × 1.1 = 22 cm
So, the radius of curvature should be 22 cm for each face of lens.
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