A double convex lens $(R_{1} = R_{2} = 100 c m)$ having focal length equal to the focal length of a concave mirror is

10 cm

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20 cm

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40 cm

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15 cm

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Solution

The correct option is C 20 cm
As we know,
$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$
$= (1.5 - 1) (\frac{1}{10} + \frac{1}{10}) = \frac{1}{10} \Rightarrow f = 10 c m$
$∴$ Radius of curvature of concave mirror $= 2 f = 20 c m$

Suggest Corrections

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d

Which of the following lenses would you prefer to use while reading small letters found in a dictionary?

(a) A convex lens of focal length 50 cm

(b) A concave lens of focal length 50 cm

(d) A concave lens of focal length 5 cm

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