Question

for a convex lens of focal length 20cm where should an object be placed to get a virtual image

Answer 1

Ans.30 cm before the lens

Magnification M =-2 ( real and inverted image)

M = v / u =-2
V=-2u ....eq (1)

From lens formula
1/f = 1/v - 1/u  .....eq (2)

Given f = 20cm

On substitution of eq (1) in (2)  we get  U = - 30 cm  ( it is negative from the sign convention)

I Hope this helps you

Answer 2

For a convex lens of focal length 20 cm where should an object be placed to get a virtual image