Question

At what distance in front of a concave mirror of focal length 30 cm should you place your face in order to obtain an image with a magnification of two?

• A. 30cm
• B. -15cm
• C. 45cm
• D. 15cm

Answer 1

The correct option is B

-15cm

Given m=2

f=-30cm

u=?

m=- $\frac{v}{u}$

v=-2u

we know that, $\frac{1}{f}$$=$$\frac{1}{u}$+$\frac{1}{v}$

$\frac{1}{(-30)}$=$\frac{1}{u}$+$\frac{1}{(-2u)}$

$\frac{1}{\left(30\right)}$=$\frac{1}{\left(2u\right)}$-$\frac{1}{u}$

$\frac{-1}{2u}$=$\frac{1}{\left(30\right)}$

u=-15cm