Question

An object s placed at a distance of 12cm from a convex lens. If a real image is formed at a distance of 24cm from the lens. Calculate the magnification.

Answer 1

I totally disagree with the other two answers given. They are incomplete and contain errors. Here is the proper solution:

A concave mirror is able to provide TWO magnified images: one REAL and the other VIRTUAL. The question does not say which type the image is. We shall consider it REAL first. Do = 12 cm, then Di = 24 cm (2 X mag). First answer is: Di = 24 cm.

Now we calculate the focal length of the lens: 1/f = 1/Do + 1/Di → 1/f = 1/12 + 1/24 → f = 8 cm.

Now we calculate Di for the virtual case. 1/8 = - (2/Di) - 1/Di → Di = - 24 cm, the negative sign showing that the image is virtual.