# A point source S and its image S1 are located on opposite sides of the lens with a focal length

July 28, 2021 | education

| **A point source S and its image S1 are located on opposite sides of the lens with a focal length F on its main optical axis. The distance from them to the lens is the same. Calculate the distance L between source S and its image S1.**

Let’s use the equation for a thin lens:

1 / F = 1 / d + 1 / f,

where

F is the focal length of the lens,

d is the distance from the object to the lens,

f is the distance from the object image to the lens;

To find:

L = f + d;

According to the condition of the problem

f = d;

Then

1 / F = 1 / (2 * d);

d = F / 2 and

f = F / 2;

We get

L = F.

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