MCQ

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

#### Options

\[f = R/2\]

- \[f = R\]
\[f = - R\]

\[f = 2R\]

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#### Solution

*f* = *R*

As from lens maker formula:

\[\frac{1}{f} = (m - 1)(\frac{1}{R_1} - \frac{1}{R_2})\]

\[ \Rightarrow \frac{1}{f} = (1 . 5 - 1)(\frac{1}{R} - \frac{1}{- R}) (\text{ As given } R_1 = R_2 = R)\]

\[ \frac{1}{f} = (0 . 5)(\frac{1}{R} + \frac{1}{R})\]

\[ \frac{1}{f} = (0 . 5)(\frac{2}{R})\]

\[ f = R\]

Is there an error in this question or solution?

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