Question

find the focal length of the convex lens if object at distance of 20 cm formed image at 30 cm?​

Answer 1

Answer:

In the given problem

Explanation:

Object distance, u = - 30 cm (from sign convention)

Image distance, v = ?

Focal length of the lens f = 20 cm

From lens formula

1f=1v−1u

120=1v−1−30

120=1v+130

1v=120−130

1v=30−2030×20=10600

v=60010=60 cm .

Magnification, m = height of the imageheight of the object =vu

M=60−30=−2.

Negative sign indicates that the image is inverted . Since, m is greater than one, the image is magnified.

Since v is positive, the image is real.

The image is formed 60 cm on the other side of the lens and it is real, inverted and magnified.

Answer 2

Answer:

object distance u= -20

image distance v =30 (assuming image is real)

1/v-1/u= 1/f

1/30+1/20= 1/f

5/60=1/f

f=12