Question

An object, located 80.0 cm from a concave lens, forms an image 39.6 cm from the lens on the same side as the object. What is the focal length of the lens?

Answer 1

Answer:

-78.4 cm

Explanation:

We can solve the problem by using the lens equation:

$\frac{1}{f}=\frac{1}{p}+\frac{1}{q}$

where

p is the distance of the object from the lens

q is the distance of the image from the lens

f is the focal length

Here we have

p = 80.0 cm

q = -39.6 cm (negative, because the image is on the same side as the object , so it is a virtual image)

Substituting, we find f:

$\frac{1}{f}=\frac{1}{80.0 cm}+\frac{1}{-39.6 cm}=-0.0128 cm^{-1}$

$f=\frac{1}{0.0128 cm^{-1}}=-78.4 cm$