Question

A concave mirror of focal length 10cm is placed at a distance of 35 cm
from a wall. How far from the wall an object be placed so that the image
formed by the mirror falls on the wall?

Answer 1

f=-10 cm
u=-35 cm
1/v=1/f- 1/u
1/v=-1/10+1/35
1/v=-7+2/70
1/v=-5/70
v=-14 cm
wall should be placed at distance of 14 cm from the mirror

Answer 2

Hello friend
___________________________________

Focal length of the mirror is 10cm

Distance from the wall is 35cm

It needs to form an image on the wall

That is the image distance will be 35cm

We need to find the object distance

By using mirror formulae

1/f = 1/v + 1/u

Using sign convention

1/-10 = 1/-35 + 1/u

-1/10 + 1/35 = 1/u

(-7+2)/70 = 1/u

-5/70 = 1/u

-1/14 = 1/u

u=-14

The object should be placed at a distance of 14cm infront of the mirror.

It means it should be placed at a distance of 35-14=21cm from the wall
___________________________________

Hope it helps!