Question

A convex lens of focal length 15 cm, is forming a real image. If the size of image is same as the size of object, then position of object and position of inage will be, respectively ______.

पर्याय

• −15 cm and −15 cm from lens

• –15 cm and +15 cm from lens

• –30 cm and +30 cm from lens

• –30 cm and –30 cm from lens

Answer 1

A convex lens of focal length 15 cm, is forming a real image. If the size of image is same as the size of object, then position of object and position of inage will be, respectively –30 cm and +30 cm from lens.

Explanation:

Given: Focal length (f) = +15 cm

h_i = h_o ⇒ Magnification |m| = 1

Since it is a real image,

m = −1

For a lens,

m = `v/u`

−1 = `v/u`

v = −u    ...(i)

By using the lens formula:

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

`1/-u - 1/u = 1/15`

`-2/u = 1/15`

u = −30 cm

v = −(−30)    ...[From equation (i)]

v = +30 cm