Question

A lens forms a real image 3 cm high of an object 1 cm high. If the separation of object and image is 15 cm , find the focal length of the lens .

Answer 1

Hii there,

● Answer -
f = 2.82 cm

● Explaination -
# Given -
h1 = 1 cm
h2 = -3 cm

# Solution -
As given distance between object and image is 15 cm.
-u+v = 15
v = u+15

Magnification by convex lens is calculated by -
M = h2 / h1 = v / u
-3 / 1 = (u+15) / u
-3u = u + 15
u = -3.75 cm

Solving for v,
v = u + 15
v = -3.75 + 15
v = 11.25 cm

To calculate focal length -
1/v - 1/u = 1/f
1/11.25 - 1/(-3.75) = 1/f
f = 2.82 cm

Therefore, focal length of given convex lens is +2.82 cm.

Hope this helps you...

Answer 2

f is equal to 2.82 v is equal to 11.25