# Power of a Lens

## Trending Questions

**Q.**

Question 37

Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm. What is the nature of the lens and its power used by each of them?

**Q.**

Define 1 dioptre power of a lens.

**Q.**

How is the power of a lens related to its focal length ?

**Q.**Focal length of a convex lens of refractive index 1.5 is 2cm. Focal length of lens when immersed in a liquid of refractive index 1.25 will be

- 5cm
- 7.5cm
- 10cm
- 2.5cm

**Q.**The magnification of a concave lens is always ___.

- less than 1
- greater than 1
- equal to 1
- negative

**Q.**

Two thin lenses of power +3.5 D and -2.5 D are placed in contact. Find the focal length of the lens combination.

1cm

100cm

-16.7 m

+16.7 m

**Q.**A convex lens and a concave lens, each having same focal length of 25 cm are placed coaxially in close contact to form a combination of lenses. The power of the combination will be

- 25 dioptre
- 50 dioptre
- Zero
- Infinite dioptre

**Q.**

Question 27

How are power and focal length of a lens related? You are provided with two lenses of focal length 20 cm and 40 cm respectively. Which lens will you use to obtain more convergent light?

**Q.**

What is the power of a convex lens of focal length 0.5 m?

**Q.**What focal length should be the reading glasses of a person be for whom the least distance of distinct vision is 50 cm? ( The distance of normal vision is 25 cm)

**Q.**Power of a Convex Lens of focal length 50 cm is

- - 0.02 Dioptre
- 2 Dioptre
- - 2 Dioptre
- 0.02 Dioptre

**Q.**An object is placed at a distance of 10 cm from a convex lens of power 5 D. The distance of the image is _______

- -20cm
- 40cm
- 45cm
- -10cm

**Q.**

Which type of lens has (a) a positive power, and (b) a negative power ?

**Q.**

The image of an object placed in the air formed by a convex refracting surface is at a distance of $10m$ behind the surface. The image is real and is at ${2}^{nd}/3$ of the distance of the object from the surface. The wavelength of light inside the surface is$2/3$ times the wavelength in air, The radius of the curved surface is $x/13m.$The value of ‘x’ is _______.

**Q.**

Define the term power of a lens. In what unit is it expressed ?

**Q.**A doctor has prescribed a corrective lens of 2.0 D to a person. Which of the following statement/s can be associated with this data?

- The corrective lens is of converging nature.
- The corrective lens is of diverging nature.
- The lens prescribed is used to correct myopia.
- The lens prescribed is used to correct hypermetropia.

**Q.**

The focal length of a lens is (i) positive, (ii) negative. In each case, state the kind of lens.

**Q.**

For which colour, the refractive index of glass is maximum?

red

violet

green

yellow

**Q.**What type of lens is used in a camera to focus the image?

- Concave lens
- Convex lens

**Q.**

The maximum magnifying power of a convex lens of focal length 5 cm can be :

(a) 25 (b) 10

(c) 1 (d) 6

**Q.**

Question 1

A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power

(a) + 0.5 D

(b) – 0.5 D

(c) + 0.2 D

(d) – 0.2 D

**Q.**Two thin lenses of focal lengths, f1 and f2 are in contact and coaxial. The combination is equivalent to a single lens of power:

- f1+f2
- f1f2f1+f2
- 12(f1+f2)
- f1+f2f1f2

**Q.**

Which lens has more power : a thick lens or a thin lens ?

**Q.**A convex lens is of focal length 10cm,

then what is its power?

**Q.**In a compound microscope the intermediate image is :

- Real, inverted and magnified
- Virtual, erect and magnified
- Virtual, erect and reduced
- Real, erect and magnified

**Q.**

**Question 7**

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

**Q.**what is the effective focal length when one surface of of a double convex lens has been silvered

**Q.**

The power of a lens is -2.0 D. Find its focal length and its kind.

**Q.**

The power of a lens is, - 2 D. What is its focal length ?

**Q.**

A person with defective eyesight is unable to see objects clearly nearer than 1.5 m. He wants to read a book placed at a distance of 30 cm from his eyes. The type of a required lens and its focal length is

Convex lens, f = 37.5 cm

Concave lens, f = 37.5 cm

Convex lens, f = 30.5 cm

Concave lens, f = 30.5 cm