Draw a labelled ray diagram for the formation of image by a convex lens of focal length 15 cm when the object is placed at a distance of 25 cm from the lens. Determine the size of the image formed, if size of the object is 4 cm.

Explanation

Convex lines
f = + 15 cm , u = - 25 cm , ` h_(1) = 4 ` cm ltbr. ` 1/v - 1/u= 1/f Rightarrow 1/v - 1/(-25) = 1/25`
` 1/v + 1/25 =1/15 Rightarrow 1/v = 1/15 - 1/25 = (5-3)/75 = 2/75`
` v = 75/2 = 37.5 cm`
` h_(2)/h_(1) = v/u Rightarrow h_(2)/4 = 75/(2 xx (-25))`
` h_(2) = (-75)/(2 xx 25) xx 4 = - 6 cm`
A real and inverted image of 6 cm is formed at 37.5 cm away form the lens on the other side of the object.