Question

A power of convex lens is + 4.0 D. At what distance should the object from the lens be placed to obtain its real and inverted image of the same size on the screen? Solve the given problem. (Ans: u = 50 cm. At the centre of curvature of lens)

Answer 1

Power of lens = + 4 D.

∵ Power = 1/Focal length

⇒ Focal length = 1/Power

∴ f = 1/4

⇒ f = 0.25 m.

Now, As per as the Question, Size of the Image is same as that of Size of Object.

∴ Linear magnification is 1.

Also,

m = v/u

⇒ v = u (since, m is 1).

Using the Lens formula,

1/f = 1/v - 1/u

⇒ 1/0.25 = 1/u - 1/(-u) [Object distance will be negative].

⇒ 100/25 = 1/u + 1/u

⇒ 4 = 2/u

⇒ u = 2/4

∴ u = 0.5 m.

∴ u = 50 cm.

Thus, object will be placed at a distance of 50 cm from lens.

Hope it helps.

Answer 2

Answer:

u= -50cm

EXPLAINATION:-