Lens Maker's Formula

Q.

A focal length of a lens is 10 cm. What is the power of a lens in dioptre?

Q.

The power of a lens is $–2D$. What is its focal length?

Q. The focal length of a biconvex lens is $f$. If we cut the lens into two equal pieces as shown in the figure, the focal length of each piece will be ___.

1. $\frac{f}{2}$
2. $0$
3. $f$
4. $2f$

Q.

Calculate the focal length of a lens, if its power is $–2.5D$.

Q.

The lens shown in figure is made of two different materials. A point object is placed on the principal axis of this lens. How many images will be obtained?

1. One
2. Two
3. Zero
4. Four

Q.

The refractive index of diamond with respect to glass is $1.6$ and absolute the refractive index of glass is $1.5$. Find the absolute refractive index of diamond.

Q.

A biconvex lens has a focal length $\frac{2}{3}$ times the radius of curvature of either surface. The refractive index of the lens material is :

Q.

For a plane convex lens $(\mu =1.5)$ having radius of curvature $10cm$. It is silvered on its plane surface. Find focal length after silvering.

1. $10cm$

2. $20cm$

3. $15cm$

4. $25cm$

Q.

Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R.

Assertion A: For a simple microscope, the angular size of the object equals the angular size of the image.

Reason R: Magnification is achieved as the small object can be kept much closer to the eye than $25cm$ and hence it subtends at a large angle.

In light of the above statements, choose the most appropriate answer from the options given below:

1. Both A and R are true but R is NOT the correct explanation of A

2. Both A and R are true and R is the correct explanation of A

3. A is true but R is false

4. A is false but R is true

Q. The focal length of a concave lens is 40 cm. If an object is kept at a distance of 60 cm from the lens, find out the distance of the image formed.

1. +20 cm
2. +24cm
3. -20 cm
4. -24 cm

Q.

The focal length of a converging lens is measured for the violet, green and red colours. It is respectively $f_v,f_g,f_r$. We will find that

1. $f_g=f_r$

2. $f_v>f_r$

3. $f_v<f_r$

4. $f_g>f_r$

Q. A thin lens of refractive index 1.5 has a focal length of 15 cm in air. When the lens is placed in a medium of refractive index 4/3, its focal length will become

1. 20 cm
2. 30 cm
3. 45 cm
4. 60 cm

Q.

The focal length of a convex lens is $50$ cm. What is its power?

Q. The radius of curvature for a convex lens is 40 cm. If its refractive index is 1.5, the focal length will be __________.

1. 40 cm
2. 80 cm
3. 60 cm
4. 20 cm

Q.

What is meant by the power of a lens? Write its SI unit. A student uses a lens of the focal length of $40\mathrm{cm}$ and another of $20\mathrm{cm}$. Write the nature and power of the lens.

Q.

A plano-convex lens is made of glass of refractive index 1.5. The radius of curvature of its convex surface is R. Its focal length is ___.

Q.

The biconvex lens has a focal length $f$. It is cut perpendicular to the principal axis passing through the optical center, then the focal length of each half is

1. $f$

2. $f/2$

3. $3f/2$

4. $2f$

Q.

The lens shown in fig. is made of two different materials. A point object is placed on the principal axis of this lens. How many images will be obtained?

Q. A thin biconvex lens is made with a material having refractive index $\mu =1.5$. It is dipped in water $(\mu =1.33)$. It will behave like

1. A converging lens
2. A diverging lens
3. A rectangular slab
4. A prism

Q. A ray directed towards the centre of curvature of a convex mirror retraces the path after reflection.

1. True
2. False

Q.

Focal length of a convex lens will be maximum for

1. blue light

2. green light

3. yellow light

4. red light

Q.

The lens shown in the figure is made of two different materials. A point object is placed on the principal axis of this lens. How many images will be obtained?

Q. Lens maker's formula derivation

Q. A thin biconvex lens having refractive index 1. 5 is dipped in water having refractive index 1.33. It will behave like a .

1. convergent lens
2. divergent lens
3. plane mirror

Q.

The power of a lens is $–2D$. What is its focal length?

1. $-50cm$

2. $50cm$

3. $5cm$

4. $-5cm$

Q.

Find the focal length of a lens of power $+4.0D$. What type of lens is this?

Q.

Question 6
Twinkling of stars is due to atmospheric

(a) dispersion of light by water droplets
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds

Q.

The refractive index of diamond with respect to glass is $1.6$ and absolute the refractive index of glass is $1.5$. Find the absolute refractive index of diamond.

Q. A person stands straight in front of a convex mirror at a distance of 20 m away from it. He sees his erect image whose length is 1/5th of his real height. Find the focal length of the convex mirror.

1. 4 m
2. 6 m
3. 5 m
4. 8 m

Q.

Question 18 (c)
A person needs a lens of power –4.5 D for correction of her vision.
What is the nature of the corrective lens?