Question

When an object is placed at a distance of 15cm from a convex lens a real image is formed at a distance of 30cm. What is the focal length of the lens?? ​

Answer 1

Answer:

1/f = 1/u - 1/v

1/f = 1/15 - 1/30

1/f = 2-1/30

1/f = 1/30

•°• f = 30 cm

•°• Focal length of the lens is 30 cm.

Answer 2

We know that focal length of a convex lens is positive

f=+ve

Image distance is negative

u=-15cm

As the image formed is real and inverted images formed on the opposite side of the lens and image distance is positive

v=30cm

As per the formula,

1/f=1/v-1/u

1/f=1/30-(-1/15)

1/f=30+15/450

f=450/45

f=+10cm

I hope this helps