Lens Formula and Magnification

Q. Select the correct lens formula where $u,v,f$ have standard meanings.

1. $\frac{1}{v}-\frac{1}{u}-\frac{1}{f}=0$
2. $\frac{1}{u}-\frac{1}{v}-\frac{1}{f}=0$
3. $\frac{1}{u}+\frac{1}{v}-\frac{1}{f}=0$
4. $\frac{1}{v}+\frac{1}{v}+\frac{1}{f}=0$

Q. An object is placed at a distance of 30 cm from a concave lens of focal length 15 cm. List four characteristics (nature, position, etc) of the image formed by the lens.

[2 Marks]
[CBSE 2017]
[Image formation by spherical lens]

Q. In a lens, if
$u$ - distance of object from lens
$v$ - distance of image from lens
$h_i$ - height of image
$h_o$ - height of object. Compare the following quanties.

A	B
$\frac{h_i}{h_o}$	$\frac{v}{u}$

1. $A>B$

2. $B>A$

3. $A=B$

4. $A=2B$

Q.

If object is placed at 60 cm before lens I focal length of lens I is 30 cm and focal length of lens II is 45 cm. find out the image distance formed by lens II from O?

1. 180 cm

2. 90 cm

3. 120 cm

4. 60 cm

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. $–10cm$

2. $+20cm$

3. $+15cm$

4. $–25cm$

Q.

Choose the position of object, kept before a lens to form image of same size as that of the object.

1. concave lens, at F₁

2. concave lens, at 2F₁

3. convex lens, at 2F₁

4. convex lens, at F₁

Q.

A $10cm$ tall candle when viewed using a convex lens appears to be $20cm$ tall as shown in the below figure. The focal length of the lens is $5cm$. What is the object distance?

1. $10cm$

2. $5cm$

3. $20cm$

4. $2.5cm$

Q.

If the power of a lens is + 0.1 D and if an object of size 2 cm is placed 5 cm before the lens. Find the size and position of the image formed?

1. Image size = 4 cm, image distance = – 10 cm

2. Image size = 10 cm, image distance = – 4 cm

3. Image size = 5 cm, image distance = 4 cm

4. Image size = 4 cm, image distance = 5 cm

Q. A lens forms a virtual, erect and diminished image of an object placed at a distance of $2m$ from it. The size of the image formed is half of that of the object. Which of the following statement is correct regarding the nature and focal length of the lens?

1. Concave lens, $f=2m$

2. Convex lens, $f=2m$

3. Concave lens, $f=-2m$

4. Convex lens, $f=-2m$

Q. Object is placed on the principal axis of a concave mirror. Its real image is formed at a point which is 10 cm more distant from the mirror than the object. If focal length of mirror 12 cm. The magnification of the image is:

1. 2
2. 1.5
3. 1
4. 2.5