Experiment 32 - Luke Wyatt Experiment 32 22/11/06...

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Luke Wyatt Experiment 32 22/11/06 Experiment 32: Measurement of focal length of converging lens by plane mirror Risk Assessment The Experiment had a very low risk. These are outlined below. 1. Electrical – due to the use of mains electricity 2. Thermal – due to the heat conducted by the light box 3. Mechanical – due to use of lenses that could shatter A: By using an illuminated object Experiment Aim The aim of the experiment was to measure the focal length of a converging lens using the light reflected from a mirror, and to determine the uncertainty in focusing the image, and the power of the lens. Below is a list of all the apparatus which was used in the experiment Illuminated object (crosswires) Converging lens Meter rule Lens holder Below is a diagram of how the apparatus was set up in order to collect the results. 1of 9
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Luke Wyatt Experiment 32 22/11/06 Theory The power of a lens is given by . 1 f P = Lenses with a short focal length, thus have a higher power. The unit of P is the ‘dioptre’, symbol D. A converging lens of f=1metre has a power of +1D. A diverging lens with f=10cm=0.1metre has a power of -10D. Results (1) (2) (3) (4) (Avg.) Distance f from centre of lens to object ( 2 10 - m) 19.4 ± 0.0 5 19.7 ± 0.0 5 20.1 ± 0.0 5 19.4 ± 0.0 5 19.7 ± 0.0 5 20.2 ± 0.0 5 19.5 ± 0.0 5 19.8 ± 0.0 5 20.2 ± 0.0 5 19.3 ± 0.0 5 19.6 ± 0.0 5 20 ± 0.05 19.4 ± 0.0 5 19.7 ± 0.0 5 20.1 ± 0.0 5 Calculation Average focal length = sum of focal lengths Number of values for focal length Average focal length = 19.7+19.7+19.8+19.6 = 19.7x 2 10 - m 4 = 19.7cm 19.7x10 ֿ ²m For the Power of the lens D f P 1 . 5 .... 0761 . 5 x10 7 . 19 1 1 2 - = = = = Power of converging lens = 5.1D Order of accuracy Error in focal length = m m m m 2 - min max x10 35 . 0 2 4 . 19 1 . 20 2 = - = - ± Error in Power = m m m m 2 - min max x10 085 . 0 2 98 . 4 15 . 5 2 = - = - ± 2of 9 Min Focal Point Max
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Luke Wyatt Experiment 32 22/11/06 Errors In the experiment there were some errors in recording the results. Firstly was the error in measurement for each value. This was because the scale used went up in millimetres and the values record had to be rounded up to the nearest millimetre which gave a reading error of 0.5mm or 5x 4 10 - m. There was also an error in the zero setting of the scale. This was because the illuminated object was slightly sunken into the light box and the meter rule only started from the outside of the box giving a error of 1mm or 1x 3 10 - m. there was also a error in setting the zero of the scale at the centre of the lens,
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Experiment 32 - Luke Wyatt Experiment 32 22/11/06...

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