7.4 - Measuring Focal Length

Measuring the focal length of a converging lens

• A converging (convex) lens brings parallel rays to a focus at the principal focus.
• Light from a distant object arrives as rays that are effectively parallel, so a sharp image forms one focal length from the lens.
• Apparatus - A converging lens, a screen (white card), and a ruler or metre stick.

Method

1. Set up the lens facing a distant object, with a screen behind the lens.
2. Move the screen back and forth until a clear, sharp, inverted image of the distant object forms on it.
3. Measure the distance from the centre of the lens to the screen. This distance is the focal length.
4. Repeat and calculate a mean to improve reliability.

Findings

A more strongly converging lens (with more curved surfaces) focuses rays over a shorter distance, so it has a shorter focal length.