Question

A bright object 50 mm high stands on the axis of a concave mirror of focal length 100 mm and at a distance of 300 mm from the concave mirror. How big will the image be ?

Answer 1

Height of the object (h) = 50 mm
Focal lenght of the mirror (f) = -100 mm
Object distance (u) = -300 mm
1/v + 1/u = 1/f
1/v = 1/f - 1/u
1/v = 1/(-100) - 1/(-300)
1/v = -1/100 + 1/300
1/v = (-3 + 1)/300
1/v = -2/300
v = 300/-2
v = -150 mm
Therefore, the image will be formed 150 mm away from the pole.
magnification (m) = h '/h = - v/u
h ' = - (v)(h)/u
h ' = - (-150)(50) / (-300)
h ' = -7500/300
h ' = -25 mm
Therefore the height of the image will be 25 mm