# A Double Convex Lens Has Two Surfaces of Equal Radii R and Refractive Index M = 1 ⋅ 5 - Science

MCQ

A double convex lens has two surfaces of equal radii R and refractive index $m = 1 \cdot 5$

#### Options

• $f = R/2$

• $f = R$
• $f = - R$

• $f = 2R$

#### Solution

f = R
As from lens maker formula:
$\frac{1}{f} = (m - 1)(\frac{1}{R_1} - \frac{1}{R_2})$
$\Rightarrow \frac{1}{f} = (1 . 5 - 1)(\frac{1}{R} - \frac{1}{- R}) (\text{ As given } R_1 = R_2 = R)$
$\frac{1}{f} = (0 . 5)(\frac{1}{R} + \frac{1}{R})$
$\frac{1}{f} = (0 . 5)(\frac{2}{R})$
$f = R$

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#### APPEARS IN

HC Verma Class 11, 12 Concepts of Physics 1
Chapter 18 Geometrical Optics
MCQ | Q 9 | Page 411