Lens Formula

Q.

A 2 cm tall object is placed perpendicular to the principal axis of a convex lens with a focal length of 10 cm. The distance of the object from the lens is 15 cm. Find the position, nature, and size of the image. Calculate the magnification of the lens.

Q.

An object is placed at a distance of 50 cm from a concave lens of a focal length of 20 cm. Find the nature and position of the image.

Q. A convex lens of focal length $10cm$ is placed at a distance of $12cm$ from a wall. How far from the lens should an object be placed so as to form its real image on the wall?

1. $20cm$
2. $50cm$
3. $30cm$
4. $60cm$

Q.

Which of the following is the lens formula?

1. $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

2. $\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$

3. $\frac{1}{f}=v+\frac{1}{u}$

4. $\frac{1}{v}=f-\frac{1}{u}$

Q. An object of height $4cm$ is placed at a distance of $24cm$ in front of the convex lens of focal length $8cm.$ The position and size of the image are respectively:

1. $12cm\&-2cm$
2. $-12cm\&2cm$
3. $12cm\&-3cm$
4. $2cm\&-2cm$

Q.

A concave lens of focal length forms an image which is $n$ times the size of the object. The distance of the object from the lens is

1. $\left(\frac{1+n}{n}\right)f$

2. $(1-n)f$

3. $(1+n)f$

4. $\left(\frac{1-n}{n}\right)f$

Q.

An object is placed 36 cm from a convex lens. A real image is formed 24 cm from the lens. Calculate the focal length of the lens. [2 MARKS]

Q.

An object is placed on the principal axis of a concave lens at a distance of 40 cm from it. If the focal length of the lens is also 40 cm, find the location of the image and the magnification. [3 MARKS]

Q.

An object $20cm$ from a convex lens forms a virtual image $15cm$ behind the lens. The focal length of the lens is

1. 8.5 cm

2. $\frac{30}{7}cm$

3. $\frac{90}{7}cm$

4. $\frac{60}{7}cm$

Q.

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens?

1. 10 cm

2. 15 cm

3. 20 cm

4. 30 cm

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

1. Convex, $2D$; Concave, $-2D$
2. Concave , $-2D$; Convex, $2D$
3. None of the above
4. Concave , $2D$; Convex, $-2D$

Q. The focal length of a convex lens is $20cm.$ If an object is kept at a distance of $100cm$ from the lens, find out the distance of the image formed from the lens.

1. $10cm$
2. $15cm$
3. $20cm$
4. $25cm$

Q. A virtual image three times the size of the object is obtained with a concave mirror of radius of curvature $36cm$. The distance of the object from the mirror is:

1. $5cm$
2. $20cm$
3. $10cm$
4. $12cm$

Q.

Example(s) of optical aids for visually challenged people is/are:

1. Magnifying lens
2. Braille writer
3. Contact lens
4. Telescopic aids

Q. For a normal eye, the near point is 25 cm from the eye lens.The distance between eye lens and retina of the eye is 2.5 cm. What is the power of the eye lens?

1. $4D$
2. $36D$
3. $40D$
4. $-44D$

Q. Size of image of an object by a mirror having a focal length of 20 cm is observed to be reduced to $\frac{1}{3}$ of its size. At what distance the object has been placed from the mirror ?

1. 80 cm
2. 60 cm
3. 50 cm
4. 70 cm

Q. An object of size 3 cm is placed at 30 cm in front of a concave mirror of radius of curvature 40 cm. Find the distance and size of image from the mirror and describe the nature of the image

Q. A projector lens has a focal length 10 cm. It throws an image of a $2cm\times 2cm$ Slide on a screen 5 meter away from the lens. Find
(i) The size of the picture on the screen and
(ii)ratio of illuminations of the slide and of the picture on the screen.

Q. If a light ray from air enters three different media in such a way that the refracted angles are \(45^{0}\), \(30^{0}\) and \(60^{0}\) respectively. Identify the right statement with respect to the optical density of these media.

Q.

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

1. $–25cm$

2. $–10cm$

3. $+20cm$

4. $+15cm$

Q. What is the power of a convex lens whose focal length is $80cm$ ?

Q. A convex lens of focal length f produce virtual image n times the size of the object then the distance of object from the lens is

1. $(n-1)f$
2. $(n+1)f$
3. $\left(\frac{n-1}{n}\right)f$
4. $\left(\frac{n+1}{n}\right)f$

Q. In astronomical telescope, the final image is formed at:

1. the least distance of distinct vision
2. the focus of objective lens
3. beyond the focal length of eyepiece.
4. the focus of the eye lens

Q. In which of the following cases a light ray suffers the maximum deviation while passing through pole?

1. 
2. 
3. 
4. 

Q. The swimming pool appears to be less deep than it actually is. Which of the following phenomenon is responsible for this?

1. Reflection of light
2. Scattering of light
3. Diffusion of light
4. Refraction of light

Q. Refraction in a parallel glass slab, implies that the incident ray is always parallel to the emergent ray.

1. True
2. False

Q.

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

1. $+20cm$

2. $+15cm$

3. $–25cm$

4. $–10cm$