A student uses a distant object (a window) to estimate the focal length of a converging lens. (a) Describe the method the student uses. [3] (b) Explain why ...

(a) Method to estimate focal length [3]

1. Hold the converging lens in front of a white screen or piece of white paper. [1]
2. Point the lens towards a distant object such as a window, tree, or building. [1]
3. Move the screen back and forth until a sharp image of the distant object is seen on it, then measure the distance from the lens to the screen with a ruler. This distance is approximately equal to the focal length. [1]

(b) Why a distant object gives a good estimate [2]

• Light rays from a very distant object are approximately parallel when they reach the lens. [1]
• Parallel rays are converged by a converging lens to a point at the principal focus. Therefore the image distance equals the focal length, giving an accurate estimate. [1]

The further the object, the closer the rays are to being truly parallel, and the more accurate the estimate. For a nearby object, the image forms further from the lens than the focal length.

(c) Focal length [1]

f = 15 cm [1]

Since the object is distant, the sharp image forms at the focal point. The measured lens-to-screen distance of 15 cm is the focal length.

(d) Source of error [1]

Difficulty in judging exactly when the image is sharpest, or parallax error when measuring the distance from the lens to the screen. [1]

The image may appear reasonably sharp over a small range of screen positions, making it hard to pinpoint the exact focal point. Also, measuring from the centre of the lens (where the principal plane sits) rather than from the lens surface introduces a small systematic error.