A concave mirror of focal length 10 cm is placed at a distance of 40 cm from a screen. How far from the screen should an object be placed, so that its image formed by the mirror falls on the screen?
PLEASE ANSWER FAST!!!!!!!

Dear Student
f = - 10 cm

v = - 40cm (As screen is 40 cm and image needs to fall on screen)

1/u + 1/v = 1/f

1/u = (-1/10) - (-1/40) = - (3/40)

on solving we get,

u = - 40/3 cm =-13.33 cm

13.33 cm from pole of mirror.
we get the value of u that is the distance of the object as -13.33 cm which means it should be placed 13.33 cm away from the mirror.
But this is the not answer we want we have to find the distance of the object from the screen not from the mirror and we know that the distance of the screen from the mirror is 35 cm
So, we will subtract the distance of the mirror from the object from the distance of the screen from the mirror:
This means,
Distance of object from the wall
=40−13.33 =  26. 6 7 cm
Distance of object from the screen = 26.67cm

Regards

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