Question

1
7. A lens forms the image of an object placed at a
distance of 45 cm from it on a screen placed at a
distance 90 cm on the other side of it. (a) Name
the kind of lens. (b) Find : (i) the focal length of
the lens, and (ii) the magnification of the image.​

Answer 1

Answer:

a) The kind of lens is convex lens.

b) The focal length of the lens is 30 cm.

c) The magnification is -2.

GIVEN :

Distance of an image from object, u = - 45 cm

Distance of the screen = 90 cm, v = 90 cm

TO FIND :

a) The kind of lens.

b) The focal length of the lens.

c) The magnification of the image.

SOLUTION :

a) The kind of lens is the convex lens.

b) Find :

i) The focal length of the lens,

Using lens formula,

→ $\dfrac {1}{v} \: - \: \dfrac {1}{u} \: = \: \dfrac {1}{f}$

→ $\dfrac {1}{90} \: + \: \dfrac {1}{45} \: = \: \dfrac {1}{f}$

→ $\dfrac {45+90}{4050} \: = \: \dfrac {1}{f}$

→ $135 f \: = \: 4050$

→ $f = \: \dfrac {4050}{135}$

→ $f \: = \: 30 cm$

ii) Magnification (m),

→ Magnification, $m \: = \: \dfrac {v}{u}$

→ $m \: = \: \dfrac {90}{-45}$