## Overview This topic covers refraction of light, and how Snell’s law relates to the wave model of light propagation. The concept of total internal reflection is studied and its application to multimode optical fibres. This topic also looks at how the introduction of monomode optical fibres has allowed for greater transmission rates and distances. ## Working Scientifically The specified practical work for this topic gives learners the opportunity to make and record observations; to keep appropriate records of experimental activities; to use appropriate analogue and digital apparatus to record a range of measurements; to use laser or light sources to investigate refraction. ## Mathematical Skills There are a number of opportunities for the development of mathematical skills in this unit. These include recognising and making use of appropriate units in calculations; estimating results; 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; changing the subject of an equation, including non-linear equations; solving algebraic equations; translating information between graphical, numerical and algebraic form; determining the slope and intercept of a linear graph; using angles in regular 2D structures; using sin, cos and tan in physical problems. ## How Science Works There are opportunities within this topic for learners to consider applications and implications of science and evaluate their associated risks. Learners can be given the opportunity to apply the concept of total internal reflection in optical fibres and use optical fibres to investigate the problem of multimode dispersion. ### Learners should be able to demonstrate and apply their knowledge and understanding of: (a) the refractive index, n, of a medium being defined as c v , in which v is the speed of light in the medium and c is the speed of light in a vacuum (b) the use of the equations: 1 1 2 2 n v n v = and 1 1 2 2 n n sin sin   = (regarded as Snell’s law) (c) how Snell's law relates to the wave model of light propagation and for diagrams of plane waves approaching a plane boundary obliquely, and being refracted (d) the conditions for total internal reflection (e) the derivation and use of the equation for the critical angle 1 2 sin C n n  = (f) how to apply the concept of total internal reflection to multimode optical fibres (g) the problem of multimode dispersion with optical fibres in terms of limiting the rate of data transfer and transmission distance (h) how the introduction of monomode optical fibres has allowed for much greater transmission rates and distances [[Specified Practical Work]] - Measurement of the refractive index of a material [[Refractive Index]]