The distance from the subject to the screen, where a clear image of the subject is obtained, 4m.

The distance from the subject to the screen, where a clear image of the subject is obtained, 4m. The images are 3 times the size of the object itself. Find the focal length of a lens

Let’s compose a system of equations for the lens:

f / d = 3;

f + d = 4.

When solving the system, we get the following answers:

d = 1;

f = 3.

System of equations for a thin lens:

1 / F = 1 + 1/3;

1 / F = 1 / f + 1 / d;

F = 3/4 = 0.75 m = 75 cm – lens focal length

Answer: 75 cm.