Ncert Exercises & Solutions

9.7 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 20cm?

Hint: Use the lens maker's formula.

Step: Find the radius of curvature of the lens.
It is given in the question that:
Refractive index of the glass = 1.55
The focal length of the lens = 20 cm
Using the lens making formula:

$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$
$\frac{1}{20} = (1.55 - 1) (\frac{1}{+ R} - \frac{1}{- R})$
$R = 0.55 \times 2 \times 20 = 22 cm$
So, the radius of curvature of the lens is 22 cm.

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