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.
Here, v=−50 cm (Since the image is virtual)
u=−20 cm
By applying the formula for lenses we have, 1v−1u=1f
⇒1f=1−50−1−20=3100
⇒f=1003=33.33 cm