Knowing the focal length of a lens helps determine its effective range of use.
Measuring the focal length of a lens does not have many practical applications outside of testing or calibrating. However, knowing the physics behind the concept of focal length helps people understand more about optics and bending light. The equations involved may seem complex, but the experiment described here makes understanding the concept and completing the calculation fairly simple.
Instructions
1. Find a window that faces outside to a landscape with moderately distant objects such as buildings or trees.
2. Hold up the lens so that it is between the window and the interior of the room.
3. Hold up a sheet of paper so that the light from the window passes through the lens and hits the paper.
4. Position the paper so that a sharp image of the objects outside of the window is formed. To get a sharp image, try moving the paper closer to, and farther away from, the lens. Which way you move it depends on the size of the lens and the objects outside, as well as the distance between the objects, the lens and the paper.
5. Measure the distance between the lens and the paper to determine the focal length of the lens.
Tags: focal length, distance between, focal length lens, length lens, lens paper