Question

An object 4 cm high is placed `40*0` cm in front of a concave mirror of focal length 20 cm. Find the distance from the mirror, at which a screen be placed in order to obtain a sharp image. Also, find the size and nature of the image formed.

Answer 1

Here, `h_(1)= 4cm,u= -40cm,f= -20cm,v=?, h_(2)=?`
From `1/u + (1)/(v) =(1)/(f), (1)/(v) = (1)/(f)- 1/u=1/-20 + 1/40 = (-1)/(40)`
or `v= -40 cm`
Therefore, screen should be kept at `40 cm` from the mirror.
From `(h_(2))/(h_(1)) = -v/u, h_(2) = -v/u xx h_(1) = (-(-40))/((-40)) xx 4 = -4 cm`
The image is real, inverted and size `= 4cm`.