Question

An object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. If object is located at 30 cm from this lens, using lens formula find out the position and nature of the image formed.

Answer 1

Given: Focal length (f) = +20 cm (positive for a convex lens)

Object distance (u) = −30 cm (Object is always placed to the left)

By using the lens formula:

`1/f = 1/v - 1/u`

`1/20 = 1/v - 1/-30`

`1/v = 1/20 - 1/30`

`1/v = 3/60 - 2/60`

`1/v = (3 - 2)/60`

`1/v = 1/60`

v = +60 cm

By using the formula for magnification:

m = `v/u`

= `60/-30`

= −2

The positive sign of v (+60 cm) indicates the image is formed on the opposite side of the lens, i.e., a real image is formed. The negative sign of m indicates the image is inverted. Since |m| > 1, the image is magnified (twice the size of the object).