A group of learners investigates the focal length of a converging lens by focusing a distant window onto a sheet of white card. Fig. 3.1 shows the arrangeme...
Question 1 Report
A group of learners investigates the focal length of a converging lens by focusing a distant window onto a sheet of white card. Fig. 3.1 shows the arrangement.
The learners repeat the measurement five times, adjusting the card each time until a sharp image of the window frame appears. Table 3.1 shows their results.
| Trial | f / cm |
|---|---|
| 1 | 14.8 |
| 2 | 15.2 |
| 3 | 14.6 |
| 4 | 15.0 |
| 5 | 15.4 |
(a) Calculate the mean focal length. Show your working.
mean f = .................. cm [2]
(b) State why light from a distant window can be treated as parallel rays for this experiment. [1]
(c) Suggest two reasons why the measured values of f are not all identical. [2]
(d) Draw a labelled ray diagram in the space below to show how three parallel rays converge after passing through a converging lens. Label the focal length f and the principal focus F. [3]
Answer Details
(a) Add the five values and divide by five: [2]
\[\text{sum} = 14.8 + 15.2 + 14.6 + 15.0 + 15.4 = 75.0 \text{ cm}\] \[\text{mean } f = \frac{75.0}{5} = 15.0 \text{ cm}\](b) The window is very far away (several metres or more). By the time light waves from a point on the window have travelled that distance, the wavefronts have become essentially flat and the rays are nearly parallel. A converging lens brings parallel rays to a focus at its focal point F, so the sharp image on the card is formed at the focal distance from the lens. [1]
(c) Two reasons why the values are not identical: [2]
- It is difficult to judge the exact card position that gives the sharpest image, so the card is placed at a slightly different distance each time.
- Parallax error when reading the ruler, or small shifts in the lens holder between trials, introduce variation.
(d) Ray diagram showing three parallel rays converging through F after passing through the lens: [3]
All three parallel rays are refracted so they pass through the principal focus F. The central ray passes through the optical centre and is undeviated. The distance from the centre of the lens to F is the focal length f.
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