Question

a convex lens forms a real image of 3/2 times height of the object at 18 cm from the lens. Calculate the focal length of the lens.

Answer 1

Answer : 7.2cm

Explanation:

✓ Real Image : v = +18cm

✓ Real Image in Convex Lens : m = -3/2 ( Inverted Image )

$m = \frac{v}{u} = - \frac{ 3}{2}$

=> u = - ( 1.5 ) x ( 18cm ) = -12cm

✓ Further :

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{18} + \frac{1}{12} = \frac{5}{36}$

=> f = 7.2cm