Question

To get an image of magnification −1 on a screen using a lens of focal length 20 cm, the object distance must be ______.

Options

• Less than 20 cm

• 30 cm

• 40 cm

• 80 cm

Answer 1

To get an image of magnification −1 on a screen using a lens of focal length 20 cm, the object distance must be 40 cm.

Explanation:

Given: Focal length (f) = 20 cm

Magnification (m) = −1 (Negative indicates a real and inverted image.)

The image is formed on the screen, meaning it is real, so the lens must be a convex lens.

By using the magnification formula:

m = `v/u`

−1 = `v/u`

v = −u

By using the lens formula,

`1/f = 1/v - 1/u`

`1/20 = 1/(-u) - 1/u`

`1/20 = -2/u`

u = −2 × 20

u = −40

The object distance must be 40 cm.