Power of a Lens

Q.

The power of a lens is -4 D. Find the focal length and type of the lens?

1. 25 cm, convex lens

2. 25 cm, concave lens

3. 40 cm, concave lens

4. 40 cm, convex lens

Q.

Question 8
Which of the following statements is true?
(a) A convex lens has 4 dioptre power having a focal length 0.25 m.
(b) A convex lens has -4 dioptre power having a focal length -0.25 m.
(c) A concave lens has 4 dioptre power having a focal length 0.25 m.
(d) A concave lens has 4 dioptre power having a focal length -0.25 m.

Q.

A beam of light travelling parallel to the principal axis of a concave lens appears to diverge from a point 25 cm behind the lens after refraction. Calculate the power of the lens. [2 MARKS]

Q. What will be the power of a concave lens whose focal length is $2m$.

1. $0.5D$
2. $-0.5D$
3. $-1D$
4. $1.5D$

Q. Primary focus of the convex lens lies on the right side of the optical centre of the lens.

1. False
2. True

Q.

How r power and focal length of a lens related? U r provided with two lenses of focal length 20cm and 40cm respectively which lens will u use to obtain more convergent light?

Q.

The ray diagram showing the correct path of the ray after passing through the lens is:

1. I, II and IV
2. I and II only
3. I, II and III
4. II and III only

Q. A boy uses spectacles of focal length 60 cm. Name the defect of vision he is suffering from which lens is used for the correction of this defect? Compute the power of this lens

1. - 1.67 D
2. 1.67 D
3. 0.067 D
4. -.0.67 D

Q. Calculate the focal length of a corrective lens having power $+2D$.

Q. Two lenses of focal lengths $f_1$ and $f_2$ are placed side by side. What is the focal length of the composite lens?

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

Q. The power of a lens is - 5D
(i) find its focal length.
(ii) name the type of lens.

Q. The power of a lens is negative. State whether it is convex or concave?

1. Concave
2. Convex
3. can't say
4. Both A & B

Q.

Negative power indicates that it is a ___.

1. concave lens

2. combination three convex lenses

3. convex lens

4. combination two convex lenses

Q.

A convex lens is a ________ and a concave lens is a _________.

1. diverging lens, converging lens

2. diverging lens, diverging lens

3. converging lens, converging lens

4. converging lens, diverging lens

Q.

Power of a convex lens of focal length $50cm$ is

1. $-0.5D$

2. $2D$

3. $0.5D$

4. $-2D$

Q. Two thin lenses of focal lengths $f_1$ and $f_2$ are placed in contact with each other. The focal length of the combination will be given by:

1. $\frac{f_1+f_2}{2}$

2. $\sqrt{f_1{f}_2}$

3. $\frac{f_1{f}_2}{f_1+f_2}$

4. $\frac{f_1{f}_2}{f_1-f_2}$

Q.

A convex lens of focal length $40cm$ is in contact with a concave lens of focal length $25cm$. The power of the combination is

1. $-6.5D$

2. $-6.67D$

3. $6.5D$

4. $-1.5D$

Q. Two thin lenses of focal lengths $f_1$ and $f_2$ are placed in contact with each other. The focal length of the combination will be given by:

1. $\frac{f_1{f}_2}{f_1-f_2}$
2. $\sqrt{f_1{f}_2}$

3. $\frac{f_1{f}_2}{f_1+f_2}$

4. $\frac{f_1+f_2}{2}$

Q. How does the power of a lens vary with its focal length?

Q. Two lenses of focal lengths $f_1$ and $f_2$ are placed side by side. What is the focal length of the composite lens?

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

Q. We can burn a piece of paper by focusing the sun rays by using a particular type of lens. Name the type of lens used for the above purpose.

Q. In a concave lens, the image formed is on the same side of the lens as the object.

1. True
2. False

Q. Minimum number of rays required to form an image of an object using a convex lens are ___.

1. 5
2. 2
3. 3
4. 4

Q. Power of a convex lens of focal length 50 cm is ___.

1. -2D
2. -0.5D
3. +0.5D
4. +2D

Q.

A lens having two spherical surfaces bulging outward is called ______________.

1. concave lens

2. plano-convex lens

3. convex lens

4. plano-concave lens

Q. The near point of an elderly person is $50$ cm from the eye. Find the power of the corrective lens that will correct his vision.

1. $+2dioptre$
2. $-2dioptre$
3. $+0.5dioptre$
4. $0.2dioptre$

Q. Match the following lenses with their names.

1. Bi-concave
2. plano-convex
3. Bi-convex

Q. Define 1 dioptre power of a lens.

Q.

If $F_1$ and $F_2$ represents the foci of the lens, then at which point will the light rays meet?

1. $F_2$

2. $2F_1$

3. $F_1$

4. $2F_2$

Q. In a concave lens, the image formed is on the same side of the lens as the object.

1. False
2. True