Derive 1 f = 1 v + 1 u formula for spherical mirrors.

From the given figure, we notice that △ A B O and △ A ’ B ’ O are similar. Therefore, A ′ B ′ / A B = O B ′ / O B (equation one) Similarly, △ A ’ B ’ F and △ O C F are similar, hence A ′ B ′ / O C = F B ′ / O F But, O C = A B Hence, A ' B ' / A B = F B ' / O F (equation two) Equating equations (1) and (2), we get O B ′ / O B = F B ′ / O F = ( O B ′ − O F ) / O F Substituting the sign convention, we get O B = - u , O B ’ = v a n d O F = f v / − u = ( v − f ) / f v f = − u v + u f o r u v = u f − v f Dividing both the sides by u v f , we get ⇒ 1 / f = 1 / v − 1 / u The above equation is known as the Lens formula. It relates the focal length of a lens with the distance of an object which is placed in front of it and the image formed of that object.

EASY

Earn 100

An object is kept at a distance of $30 cm$ from a diverging lens of focal length $15 cm$ . At what distance the image is formed from the lens? Find the magnification of the image.

Important Questions on Light – Reflection and Refraction

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

The unit of power of lens is _____.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Define power of a lens.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Find the power of a concave lens of focal length

2 m

HARD

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

20 mm

tall object is placed perpendicular to the principal axis of a convex lens of focal length

10 cm

. If the distance of the object from the lens is

15 cm

, then find the magnification of the image.

MEDIUM

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

An object of height

4.0 cm

is placed at a distance of

30 cm

from the optical centre ‘

O

’ of a convex lens of focal length

20 cm

. Draw a ray diagram to find the position and size of the image formed. Mark optical centre ‘

O

’ and principal focus ‘

F

’ on the diagram. Also, find the approximate ratio of the size of the image to the size of the object.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Find the power of a concave lens of focal length

2 m

MEDIUM

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Find the power of a concave lens of focal length

2 m

MEDIUM

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

A concave lens has a focal length of

15 cm

. At what distance should the object from the lens be placed so that it forms an image at

10 cm

from the lens? Also, find the magnification produced by the lens? What is the nature of the image?

MEDIUM

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

10 cm

tall object is placed perpendicular to the principal axis of a convex lens of focal length

12 cm

. The distance of the object from the lens is

18 cm

. Find the nature, position and size of the image formed.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length

40 cm

and another of

- 20 cm

. Write the nature and power of each lens.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

An eye specialist prescribes a power

+ 4.5 D

to a person for his glasses. What are the nature of the lens and the focal length of the lens?

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Define power of a lens. What is its unit?

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

A convex lens of focal length

40 cm

and a concave lens of focal length

50 cm

are placed in contact with each other.
Calculate (i) the power of the Combination and (ii) the focal length of the combination.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

The power of lens is $+ 5 D$ , then the focal length of the lens will be _____.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Define Power of lens. Give its unit.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Find the power of a concave lens of Focal length $2$ (two) metres.

HARD

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Derive

\frac{1}{f} = \frac{1}{v} + \frac{1}{u}

formula for spherical mirrors.

EASY

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

Define magnification of lens.

HARD

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

An object is placed at a distance $24 cm$ in front of a convex lens of focal length $8 cm$ .
(i) Calculate the distance of the image from the lens.
(ii) Calculate the magnification of the image.

MEDIUM

Science>Physics>Light – Reflection and Refraction>Refraction by Spherical Lenses

A concave lens has focal length

15 cm

. At what distance should the object from the lens be placed so that it forms an image at

10 cm

from the lens? Also find the magnification produced by the lens.

An object is kept at a distance of 30 cm from a diverging lens of focal length 15 cm. At what distance the image is formed from the lens? Find the magnification of the image.

Important Questions on Light – Reflection and Refraction

Learn and Prepare for any exam you want !

An object is kept at a distance of $30 cm$ from a diverging lens of focal length $15 cm$ . At what distance the image is formed from the lens? Find the magnification of the image.