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LEVEL: AS PHYSICS – Waves and Oscillations

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10. Superposition (15 minutes) (a) (i) What is meant by 'superposition' when the term is used literally. (1 mark) Superposition is the act of placing one figure on top of another. (ii) State the 'principle of superposition' as applied to waves. (2 marks) When two waves superpose, the total displacement at a point equals the sum of the individual displacements at that point. (b) Colours can be produced on the surface of a very thin film as a result of light reflecting from the top and bottom of the film and then interfering. This is called irridescence. You may have seen this effect if you have noticed a range of colours on oily water. (i) Explain how monochromatic light could vanish if some of the light reflected from the top of an oil slick and some reflected from the bottom. (4 marks) The light that reflected from the bottom of the oil film would travel further than the light that reflected from the top. If it travelled an extra half wavelength (or odd multiple of half wavelengths), destructive interference would occur and the light would vanish. (ii) It was noticed that when white light was incident upon an oil film of constant thickness, only blue light disappeared. Why? (3 marks) The thickness of the film must have been just right to remove blue only. The extra distance travelled by the light that reflected from the bottom surface must have been an odd multiple of half wavelengths of blue light. This would mean that only blue light would undergo destructive interference and disappear. (iii) When oil slicks spread they become thinner. Therefore oil slicks tend to get thinner towards their edges. Explain what effect this would have if an oil slick was viewed in normal daylight. (3 marks) Different depths of oil would cause destructive interference for different wavelengths of light. Therefore different wavelengths of light would disappear at various points so the oil slick would appear multi-coloured. (iv) Give two more examples of irridescence. (2 marks) Other examples include the colours seen in bubble bath bubbles or when a film of washing-up liquid forms across the open end of a glass. (Total marks 15)