Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12

# A Man Uses a Concave Mirror for Shaving. He Keeps His Face at a Distance of 25 Cm from the Mirror and Gets an Image - Physics

Sum

A man uses a concave mirror for shaving. He keeps his face at a distance of 25 cm from the mirror and gets an image which is 1.4 times enlarged. Find the focal length of the mirror.

#### Solution

Given,
Distance of the man's face (here, taken as object), u = −25 cm
According to the question, magnification, m = 1.4

$m = \frac{A'B'}{AB} = - \frac{v}{u}$

$\Rightarrow 1 . 4 = - \frac{(v)}{- 25}$

$\Rightarrow\frac{14}{10}=\frac{v}{25}$

$\Rightarrow v=\frac{25 \times 14}{10}=35 \text{ cm }$
Using equation of mirror, we get:
$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$
⇒ 1/f  = 1/35 - 1/25
$= \frac{5 - 7}{175} = - \frac{2}{175}$
⇒ f = −87.5

Hence, the required focal length of the concave mirror is 87.5 cm.

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#### APPEARS IN

HC Verma Class 11, 12 Concepts of Physics 1
Chapter 18 Geometrical Optics
Q 8 | Page 412