Some students determine the focal length of a converging lens by two different methods.
A student sets up an illuminated object, a card, a converging lens, and a plane mirror on a bench. The distance between the illuminated object and the mirror is set to 44.0 cm. The student moves the lens until a sharp image of the object is formed on the card beside the object.
A scale diagram of the setup is drawn to 15th\displaystyle \frac{1}{5}\text{th}51th scale. On this diagram, the measured distance f1 f_1\,f1 between the object and the lens is 3.0 cm. Calculate the actual distance F1F_1F1 (the focal length) between the object and the lens in the experiment.
Briefly describe a technique to obtain an image on the card that is as sharp as possible in this experiment.
Another student sets up the apparatus with a distance between the illuminated object and the lens of U=22.0 cmU = 22.0\text{ cm}U=22.0 cm, using a screen instead of a mirror. She moves the screen until a sharp image of the object appears on the screen.
The student measures the height of the object hO=1.5 cmh_O = 1.5\text{ cm}hO=1.5 cm and the height of the image on the screen as hI=3.3 cmh_I = 3.3\text{ cm}hI=3.3 cm. Describe one difficulty when measuring the height of the image on the screen in this type of experiment, and suggest an improvement to overcome this difficulty.
Calculate the value of W W\,W using the equation:
W=hOhI W = \frac{h_O}{h_I} W=hIhOCalculate a second value F2 F_2\,F2 for the focal length of the lens, using the equation:
F2=UW+1 F_2 = \frac{U}{W + 1} F2=W+1UA student suggests that F1 F_1\,F1 and F2 F_2\,F2 should be equal. State whether your results support this suggestion. Justify your statement by comparing the two values with reference to experimental accuracy.
Describe one precaution that can be taken to obtain reliable measurements of F1 F_1\,F1 and U U\,U in this experiment.
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.