Question

the two thin plano-convex lenses, each of focal length 40cm are combined to form a bioconvex lens. how far from thislens should an object be placed such that its inverted image of unit magnification is formed on the other side of the lens?

Answer 1

u = -80 cm

Explanation:

Given:

Focal length of two thin plano-convex lens f = 40 cm

Magnification, m = unit magnification = 1

Magnification is negative for convex lens while focal length is a positive figure as the image is real but inverted.

So f = +40 and m = -1

To Find: object distance u.

Solution:

We know that: m = f / (f + u)

So u = f * (1 - m) / m

Substituting the values, we get:

u = 40 * [1 - (-1)] / (-1)

Therefore u = -80 cm

The object should be placed 80 cm away from the lens to get an inverted image of unit magnification.