Question

Harish stands in front of a giant convex lens in science museum . A sharp image thats half his height is obtained on a screen placed 1.8cm behind the lens . Find the focal length of the lens

Answer 1

Focal length of the convex lens = 1.2cm.

Explanation:

Magnification m = v/u = 1/2

So u = 2v ---- (1)

Image distance v = 1.8cm

So object distance u = 2v = 2 * 1.8 = 3.6 cm

Lens formula is 1/u + 1/v = 1/f

Substituting the values, we get:

1/ 3.6 + 1/1.8 = 1/f

1+ 2 / 3.6 = 1/f

3/3.6 = 1/f

So f = 3.6 / 3 = 1.2 cm

Focal length of the convex lens = 1.2cm.