## Overview This topic introduces the wave properties of diffraction and interference. Investigation of two source interference patterns and the diffraction grating are carried out. The topic deals with coherent and incoherent sources and the conditions needed for two source interference to be observed. Stationary waves are introduced, and the differences between stationary and progressive waves investigated. ## Working Scientifically The specified practical work in this topic gives learners the opportunity to safely and correctly use a range of practical equipment and materials; to make and record observations; to present information and data in a scientific way; to use a wide range of experimental and practical instruments, equipment and techniques; to generate and measure waves, using a microphone and loudspeaker; to use a laser/light source to investigate interference/diffraction. ## Mathematical Skills There are a number of opportunities for the development of mathematical skills in this unit. These include recognising and using expressions in decimal and standard form; using calculators to handle sin x, cos x, tan x when x is expressed in degrees or radians; using an appropriate number of significant figures; making order of magnitude calculations; understanding and using the symbols: =, <, <<, >>, >, ∝, ≈, ∆; substituting numerical values into algebraic equations using appropriate units for physical quantities; plotting two variables from experimental or other data; using angles in regular 2D structures; visualising and representing 2D forms; using Pythagoras’ theorem, and the angle sum of a triangle; using small angle approximations including sin   , tan   , cos 1   for small θ where appropriate. ## How Science Works There are opportunities within this topic for learners to use appropriate methodology, including ICT to answer scientific questions and to solve scientific problems; to evaluate methodology, evidence and data, and resolve conflicting evidence; to know that scientific knowledge and understanding develops over time; to communicate information and ideas in appropriate ways using appropriate terminology. Learners can be given the opportunity to understand that scientific knowledge and understanding develops over time by considering the historical importance of Young’s experiment and how it demonstrated the wave nature of light. Learners can be given the opportunity to carry out investigational activities to determine the wavelength of light using Young’s double slits and a diffraction grating and evaluate the relative merits of both methods. ### Learners should be able to demonstrate and apply their knowledge and understanding of: (a) diffraction occuring when waves encounter slits or obstacles (b) the idea that there is little diffraction when  is much smaller than the dimensions of the obstacle or slit (c) the idea that if  is equal to or greater than the width of a slit, waves spread as roughly semicircular wavefronts, but if  is less than the slit width the main beam spreads through less than 180° (d) how two source interference occurs (e) the historical importance of Young’s experiment (f) the principle of superposition, giving appropriate sketch graphs (g) the path difference rules for constructive and destructive interference between waves from in phase sources (h) the use of a y D   = (i) the derivation and use of d n sin  = for a diffraction grating (j) the idea that for a diffraction grating a very small d makes beams (“orders”) much further apart than in Young’s experiment, and that the large number of slits makes the bright beams much sharper (k) the idea that coherent sources are monochromatic with wavefronts continuous across the width of the beam and, (when comparing more than one source) with a constant phase relationship (l) examples of coherent and incoherent sources (m) the idea that for two source interference to be observed, the sources must have a zero or constant phase difference and have oscillations in the same direction (n) the differences between stationary and progressive waves (o) the idea that a stationary wave can be regarded as a superposition of two progressive waves of equal amplitude and frequency, travelling in opposite directions, and that the internodal distance is 2  [[Specified Practical Work]] - Determination of wavelength using Young’s double slits - Determination of wavelength using a diffraction grating - Determination of the speed of sound using stationary waves