Question

6] (a) Define power of a lens.
(b) An object is kept at a distance 18 cm, 20 cm, 22 cm and 30 cm from a lens +5 D .
_ In which case or cases would you get a magnified image ?
_Which of the magnified image can you get on a screen ?
(c) List two widely used application of convex lens.

Answer 1

a) Power of a lens is the degree to which a lens can diverge or converge the light passing through it. It is the reciprocal of focal length. Its unit is dioptre or m⁻¹.

b)power of lens = 5D
P = 1/f
⇒ f = 1/P = 1/5 = 0.2m = 20cm
So focal length = 20cm

you will get a magnified image when object is between f and 2f. So you will get magnified image in case of 22cm and 30 cm.
In case of 18cm, virtual, magnified image will be formed.
In case of 20cm, image will be formed at infinity.

So you can get magnified image on a screen in case of 22cm and 30cm.

c)
-used in spectacles to help people with visual imparment
- used in telescopes, microscopes etc.