Lens Formula

Q.

Find the power of a concave lens of focal length 2 m.

Q. A 5 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm. Find the position and nature of the image formed.

1. Position of image = 60 cm, image is virtual and erect
2. Position of image = - 60 cm, image is virtual and erect
3. Position of image = 60 cm, image is real and inverted
4. Position of image = 12 cm, image is real and inverted

Q. A lens of focal length 20 cm is used to produce a ten times magnified image of a film slide on a screen. How far must the slide be placed from the lens?

Q.

An object is placed 20 cm in front of a concave mirror of focal length 12 cm. Find the position and nature of the image.

Q.

A small object is so placed in front of a convex lens of 5cm focal length that a virtual image is formed at a distance of 25 cm . Find the magnification

Q.

An object is placed at a distance of 20 cm in front of a concave lens of focal length 20 cm. Find :

(a) the position of image, and

(b) the magnification of image.

Q.

What is the power of a combination of lenses?

Q. A convex lens of focal length 30cm produces a real image 3times magnified. Find the object distance.

Q.

Question 35
The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens?

Q.

To form an image twice the size of the object, using a convex lens of focal length $20cm$, the object distance must be

1. < 20 cm

2. > 20 cm

3. < 20 cm and between 20 cm and 40 cm

4. equal to 20 cm

Q. A doctor has prescribed a corrective lens of power +2.5 D to a person. Which of the following is/are correct about the lens?

1. Focal length of the lens is 0.4 cm.
2. Focal length of the lens is 40 cm.
3. The lens is a converging lens.
4. The lens is a diverging lens

Q.

An object of $5cm$ is placed at a distance of $15cm$ from a concave mirror of focal length $10cm$. Find the position, nature and height of the image.

Q.

Why focal length of convex mirror and convex lens is positive while concave mirror and concave lens negative?

Q. A 4 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. If the distance of the object is 30 cm from the lens, which of the following statements are correct for the position and height of the image?

1. Position of the image is 15 cm
2. Position of the image is 60 cm
3. Height of the image is 8 cm
4. Height of the image is -8 cm

Q.

With respect to air, the refractive indices of water and benzene are $1.33$ and $1.50$ respectively. Calculate the refractive index of benzene with respect to water.

Q. An illuminated object lies at a distance 1.0 m from a screen. A convex lens is used to form the image of the object on a screen placed at a distance 75 cm from the lens. Find :
1) the focal length of lens
2) the magnification.

Q.

A concave lens forms the image of an object kept at a distance 20 cm in front of it, at a distance 10 cm on the side of the object. (a) What is the nature of the image ? (b) Find the focal length of the lens.

Q. An object of size 5 cm is kept at a distance of 25 cm from the optical centre of a converging lens of focal length 10 cm. Calculate the distance of image from the length and the size of the image?

Q. An object of size 1cm is placed at a distance of 15cm in front of a convex lens of focal length 10cm . Find the nature, size, location of the image

Q.

The critical angle for glass is $42°$. The corresponding angle of refraction is:

1. $0°$

2. $90°$

3. lesser than $90°$ but more than $42°$

4. no angle of refraction

Q. If the boy looks through the tube with 4 mirrors fixed at 45 degrees. How far the ball will appear from the boy?

1. 
   F - (A + E)
2. 2F
3. A + B + C + D + E
4. 
   F

Q.

An object is placed at 30 cm in front of the convex lens of focal length 10 cm. Find out the image distance from lens?

1. – 10 cm

2. + 20 cm

3. + 15 cm

4. – 20 cm

Q. A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 40 cm. Calculate the :
(a) Power of each lens
(b) Power of resulted lens
(c) Nature of resulted lens
(d) Focal length of the resulted lens
[5 MARKS]

Q.

An object is placed in front of a converging lens at a distance greater than twice the focal length of the lens. The image obtained is:

1. Real and inverted; magnified

2. Virtual and upright; magnified

3. Real and inverted; diminished

4. Virtual and upright; diminished

Q.

A point-like object is placed at a distance of $1m$ in front of a convex lens of the focal length of$0.5m$. A plane mirror is placed at a distance of $2m$ behind the lens. The position and nature of the final image formed by the system are:

1. $1m$ from the mirror, virtual

2. $2.6m$ from the mirror, virtual

3. $1m$ from the mirror, real

4. $2.6m$ from the mirror, real

Q.

13. An object is placed $6cm$ in front of a concave mirror having focal length $10cm$. The object height is $1.2cm$. Determine the location and size of image?

Q. An object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm at a distance of 20 cm from the lens. Find the position of the image.
What is the magnification of the object? [4 MARKS]

Q. A lens of focal length 10 cm is being used by a student in the laboratory as magnifying glass. His near point distance is 25 cm. What magnification is the student getting ?

Q.

Focal length of a convex lens is $20cm$. Its power is

1. $+5D$

2. $-5D$

3. $+0.05D$

4. $-0.05D$

Q.

An object of the height of 2 cm is placed perpendicular to the principal axis of a concave lens of the focal length of 12 cm. find the height of the image, if the object is placed at a distance of 8 cm from the lens?