Question

where should ab object be placed in front of concave mirror of focal lengthy 20cm so as to obtain real image two times magnified

Answer 1

Focal length, f = -20 cm

Since, a real and magnified image is formed. Let object distance be –u, image distance be –v.

The magnification is given as, m = -v/u = -2 (negative sign appears with the number ‘2’ because the image is real)

Therefore, v = 2u

Now using mirror formula,

(-1/20) = (-1/v) + (-1/u)

=> 1/20 = 1/2u + 1/u

=> 1/20 = 3/2u

=> u = 30 cm (Objet distance = -u = -30 cm)

=> v = 60 cm (Image distance = -v = -60 cm)

So, the object must be placed at a distance 30 cm from the pole of the mirror.