Thin lenses: It is desired to project the image of an object four times its actual size using a lens of focal length 20 cm. How far from the lens should the object be placed?

Question

Quizplus · Accepted Answer

According to the thin lens equation:

1/s + 1/s' = 1/f

where s is the object distance, s' is the image distance, and f is the focal length of the lens.

The magnification of the image, M is given by:

M = -s'/s

where the negative sign indicates an inverted image.

We are given that M = 4, f = 20 cm. Substituting these values into the magnification formula gives:

4 = -s'/s

or

s' = -4s

Substituting these values into the thin lens equation, we get:

1/s + 1/-4s = 1/20

Solving for s gives:

s = 25 cm

Therefore, the object should be placed 25 cm away from the lens. The answer is choice D.

Thin Lenses: It Is Desired to Project the Image of an Object