A student conducts an experiment to determine the focal length of a thin converging lens. The table below shows two sets of measurements of the object distance UUU, the image distance VVV, and the total distance DDD:
TrialU/cmV/cmD/cm124.036.060.0222.040.062.5 \begin{array}{|c|c|c|c|} \hline \text{Trial} & U / \text{cm} & V / \text{cm} & D / \text{cm} \\ \hline 1 & 24.0 & 36.0 & 60.0 \\ \hline 2 & 22.0 & 40.0 & 62.5 \\ \hline \end{array} Trial12U/cm24.022.0V/cm36.040.0D/cm60.062.5Use the results from the first trial to calculate a value f1 f_1\,f1 for the focal length of the lens. Use the equation f1=UVD\displaystyle f_1 = \frac{UV}{D}f1=DUV.
Use the results from the second trial to calculate a value f2 f_2\,f2 for the focal length of the lens. Use the equation f2=UVD\displaystyle f_2 = \frac{UV}{D}f2=DUV.
Calculate the average value favgf_{\text{avg}}favg for the focal length of the lens. Give your answer to a suitable number of significant figures for this experiment.
Suggest two practical reasons why it may be difficult to obtain highly reliable results in this lens experiment.
A student states that a more reliable value for the focal length is obtained if more values of UUU, VVV, and D D\,D are collected, enabling a graph of UV UV\,UV against D D\,D to be plotted.
Suggest a suitable number of values for UUU.
Suggest a suitable range of values for UUU.
216 exam-style questions on CIE IGCSE Physics 3.2 Light, covering 3.2.1a Reflection of light, 3.2.1b Images in a plane mirror, 3.2.2 Refraction of light, 3.2.2a Refraction of light, 3.2.2b Total internal reflection, 3.2.2c Refractive index and critical angle, 3.2.2d Optical fibres, 3.2.3a Converging and diverging lenses, 3.2.3b Ray diagrams and the nature of images, 3.2.3c Virtual images and the magnifying glass, 3.2.3d Correcting long-sightedness and short-sightedness, and 3.2.4 Dispersion of light. Each one has a worked solution and a mark scheme showing where the marks go.