Question

An object is placed on the principal axis of a concave lens at a distance of 40 cm from it. If the focal length of the lens is also 40 cm. The location of the image is :

• A. -13.33 cm
• B. 13.33 cm
• C. -20 cm
• D. 20 cm

Answer 1

The correct option is C -20 cm

For a concave lens focal length f is negative, i.e. f = - 40 cm.

By sign convention, object is placed on the left of the lens, so u = - 40 cm.

Using the lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$, we get

$\frac{1}{v}-\frac{1}{-40}=\frac{1}{-40}$

$v=-20cm$

The image is formed 20 cm from the lens. Negative sign shows that the image is formed on the same side of the lens as the object.