Calculate the focal length of a convex lens which produces a virtual image at a distance of 50 cm of an object placed 20 cm in front of it.
Given
Image distance, v=50cm
Object distance u=20cm
Since, the image is virtual it will be formed on the same side of the object.
From lens formula,
1f=1v−1u
1f=1−50−1−20
=3100
f=1003cm= focal length