Question

at what distance from a concave mirror of focal length 30 should an object placed so that size of image is size of obj​

Answer 1

Given:-

focal length= -30

u= -x

real=same size = v= -ve

Magnification,

$m = \frac{v}{u} = > v = m \times u = 3 \times ( - x)$

$= > - 3x$

From mirror formula,

$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$

We get,

$\frac{1}{ - 30} = \frac{1}{ - 3x} + \frac{1}{ - x} = > \frac{1}{30}$

$= \frac{1 + 30}{3x}$

$= > \frac{3x}{4} = 3 = > x = 40$

So object should be placed at 40 cm to the mirror.