Physics 2021 JAMB Past Questions - Question 24:  A converging lens has a focal length of 20cm. The lens forms a real image of the same size as the object when the object distance from the lens is?

Correct Answer

C

Explanation

To find the object distance from the lens when a real image of the same size as the object is formed, we can use the lens equation:

\[\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}\]

Where:
\(f\) = focal length of the lens
\(d_o\) = object distance from the lens
\(d_i\) = image distance from the lens

Given that the focal length (\(f\)) is 20 cm and the image is formed at the same size as the object, we can use the fact that the magnification (\(M\)) for a real image formed by a converging lens is negative and equal to 1 to solve for the object distance (\(d_o\)).

The magnification equation is given by:

\[M = -\frac{d_i}{d_o}\]

Since the magnification is 1, we have:

\[1 = -\frac{d_i}{d_o}\]

This implies that \(d_i = -d_o\).

Substituting \(d_i = -d_o\) into the lens equation, we get:

\[\frac{1}{f} = \frac{1}{d_o} - \frac{1}{d_o}\]

Solving for \(d_o\), we find:

\[\frac{1}{f} = \frac{1}{d_o}\]

\[d_o = f\]

So, the object distance from the lens when a real image of the same size as the object is formed is 20 cm.