# Solve Numerical example. A person uses spectacles of ‘number’ 2.00 for reading. Determine the range of magnifying power (angular magnification) possible. - Physics

Numerical

Solve Numerical example.

A person uses spectacles of ‘number’ 2.00 for reading. Determine the range of magnifying power (angular magnification) possible. It is a concavoconvex lens (n = 10.5) having a curvature of one of its surfaces to be 10 cm. Estimate that of the other.

#### Solution

For a single concavo-convex lens, the magnifying power will be the same as that for a simple microscope As the number represents the power of the lens,
P = 1/"f" = 2 ⇒ f = 0.5 m.
∴ Range of magnifying power of a lens will be,
Mmin = "D"/"f"=0.25/0.5 = 0.5

and Mmax = 1+"D"/"f" = 1 + 0.5 = 1.5

Given that, n = 1.5, |R1| = 10 cm
f = 0.5 m = 50 cm
From lens maker’s equation,
1/"f"=("n"-1)(1/"R"_1-1/"R"_2)

1/50=(1.5-1)(1/10-1/"R"_2)

∴ 0.04 = (1/10-1/"R"_2)

∴ 1/"R"_2=1/10-0.04=3/50

∴ R2 = 50/3 cm

The range of magnifying power is from 0.5 to 1.5 radius of curvature of other surface is 50/3 cm.

Concept: Refraction at a Spherical Surface and Lenses
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#### APPEARS IN

Balbharati Physics 11th Standard Maharashtra State Board
Chapter 9 Optics
Exercises | Q 4. (xii) | Page 187