Question

A real image
$\frac{4}{5}$
Size of object is formed 18cm from a lens. Calculate the focal length of the lens.​

Answer 1

Given:-

• Image Distance (v) = +18 cm

• Height of object (ho)= 5

• Height of image (hi) = -4

To Find:-

• Focal Length of the lens

Solution:-

As we know that

→ Magnification = hi/ho = v/u

→ m = hi/hu = v/u

Put the value we get

→ -4/5 = 18/u

→ -4u = 18×5

→ u = -90/4

→ u = - 22.5cm

And Now by using lens formula

→ 1/v - 1/u = 1/f

Substitute the value we get

→ 1/18 -1/(-22.5) = 1/f

→ 1/f = 1/18 + 1/22.5

→ 1/f = 9/90

→ f = 90/9

→ f = 10cm

∴ The Focal length of the lens is 10cm.