Power of a Lens

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

1. 5cm
2. 7.5cm
3. 10cm
4. 2.5cm

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

1. less than 1
2. greater than 1
3. equal to 1
4. 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.

1. 1cm

2. 100cm

3. -16.7 m

4. +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

1. 25 dioptre
2. 50 dioptre
3. Zero
4. 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

1. - 0.02 Dioptre
2. 2 Dioptre
3. - 2 Dioptre
4. 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 _______

1. -20cm
2. 40cm
3. 45cm
4. -10cm

Q.

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

Q.

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

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. 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?

1. The corrective lens is of converging nature.
2. The corrective lens is of diverging nature.
3. The lens prescribed is used to correct myopia.
4. 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?

1. red

2. violet

3. green

4. yellow

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

1. Concave lens
2. 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, $f_1$ and $f_2$ are in contact and coaxial. The combination is equivalent to a single lens of power:

1. $f_1+f_2$
2. $\frac{f_1{f}_2}{f_1+f_2}$
3. $\frac{1}{2}(f_1+f_2)$
4. $\frac{f_1+f_2}{f_1{f}_2}$

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 :

1. Real, inverted and magnified
2. Virtual, erect and magnified
3. Virtual, erect and reduced
4. 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

1. Convex lens, f = 37.5 cm

2. Concave lens, f = 37.5 cm

3. Convex lens, f = 30.5 cm

4. Concave lens, f = 30.5 cm