Question

The lens labelled (A) has radii 12.5cm and 16.7cm. Using the shape of the lens to assign the signs of the radii, calculate the focal length of the lens. Give your answer with the correct positive or negative sign using the usual convention for lenses.

Answer 1

The focal length for given convex lens = 0.0714 m

Step-by-step explanation:

Given data

Radius (
`R_(1)`) = 12.5 cm

Radius (
`R_(2)`) = 16.7 cm

Since the radius of curvature is positive so these are the convex lens.

Now focal length is given by

⇒
`(1)/(f)` =
`(1)/(R_(1) )` +
`(1)/(R_(2) )`

⇒ Put the values of
`R_(1)` and
`R_(2)` in the above formula we get,

⇒
`(1)/(f)` =
`(1)/(12.5)` +
`(1)/(16.7)`

⇒
`(1)/(f)` = 0.13988

⇒ f =
`(1)/(0.13988)`

⇒ f = 7.14 cm

⇒ f = 0.0714 m

Thus the focal length for given convex lens = 0.0714 m