Question

a concave mirror of focal length 20cm is placed at a distance of 50cm from the wall how far from the wall should an object be placed to form it's real image on the wall

Answer 1

according to the question, f = -20cm
v =50cm.
u = ?


using mirror formula,
1/v +1/u =1/f
1/50 + 1/u = 1/-20
1/u =1/-20 -1/50
1/u = -5 -/ 100
u = - 100/7
u~ 14.2