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1 of 34 © Boardworks Ltd 2009 2 of 34 © Boardworks Ltd 2009 What is optics? Optics is the study of the nature and behaviour of visible light, along with its interactions with matter. These interactions include reflection, refraction, total internal reflection and diffraction. This definition can be extended to encompass the family of waves to which light belongs: the electromagnetic spectrum. 3 of 34 © Boardworks Ltd 2009 Properties of EM waves Electromagnetic (EM) waves travel as oscillations in electrical and magnetic fields, and can transmit energy through a vacuum. They are always transverse waves. electric field magnetic field In a vacuum, EM waves travel at the speed of light (299,792,458 ms-1) but they slow down in different substances. 4 of 34 © Boardworks Ltd 2009 The EM spectrum 5 of 34 © Boardworks Ltd 2009 Lasers as light sources Lasers are often used as light sources because their light is: monchromatic – emitted with only one wavelength coherent – all waves are either exactly in phase or display a constant phase difference collimated – a narrow, approximately parallel, beam Other light sources, such as the Sun and light bulbs, are polychromatic, incoherent and uncollimated. 6 of 34 © Boardworks Ltd 2009 Electromagnetic spectrum: a summary 7 of 34 © Boardworks Ltd 2009 8 of 34 © Boardworks Ltd 2009 What is refraction? Refraction is the change of direction a light ray undergoes when it enters a medium with a different optical density. Light travels at different speeds in materials with different optical densities, and the change in direction occurs due to the change in the speed of the light. In a vacuum, light travels at 299,792,458 ms-1. 9 of 34 © Boardworks Ltd 2009 Why does refraction occur? As a light ray enters a medium that is more optically dense, it slows down and bends towards the normal. As a light ray enters a medium that is less optically dense, it speeds up and bends away the normal. In this diagram: i1 normal air glass r1 normal i2 r2 i1 > r1 i1 = r2 10 of 34 incident ray i2 < r2 r1 = i2 refracted ray © Boardworks Ltd 2009 Investigating refraction 11 of 34 © Boardworks Ltd 2009 Refractive index The speed of light in a particular substance is expressed as the refractive index (n) of that medium. refractive index = of substance (ns) medium speed of light in vacuum (c) speed of light in substance (cs) speed of light (ms-1) refractive index (n) vacuum air 299,792,458 299,702,547 1 1.0003 water 224,900,569 1.333 glass 198,538,051 1.51 diamond 123,933,393 2.419 12 of 34 © Boardworks Ltd 2009 Snells’ law of refraction Refractive indices can be used to make predictions about refraction. θ1 normal medium 1: refractive index = n1 θ2 medium 2: refractive index = n2 Snell’s law of refraction states: 13 of 34 n1sinθ1 = n2sinθ2 © Boardworks Ltd 2009 Refraction calculations 14 of 34 © Boardworks Ltd 2009 15 of 34 © Boardworks Ltd 2009 Total internal reflection 16 of 34 © Boardworks Ltd 2009 Finding the critical angle 17 of 34 © Boardworks Ltd 2009 Optical fibres and TIR 18 of 34 © Boardworks Ltd 2009 Structure of optical fibres 19 of 34 © Boardworks Ltd 2009 TIR and the critical angle: a summary 20 of 34 © Boardworks Ltd 2009 21 of 34 © Boardworks Ltd 2009 What is diffraction? Diffraction is the spreading out or bending of waves as they pass through a gap or around an obstacle. All types of waves can be diffracted, but the amount depends on the ratio of the wavelength to the size of the opening or obstacle. Diffraction is greatest when the wavelength is approximately the same as the width of the gap. 22 of 34 © Boardworks Ltd 2009 When light passes through a narrow slit, a pattern of alternate bright and dark fringes is produced. This is a single slit diffraction pattern. The intensity of the fringes against distance from the centre can be plotted on a graph. 23 of 34 intensity Single slit diffraction distance from centre © Boardworks Ltd 2009 Single slit diffraction pattern intensity The central maximum is twice as wide as the other fringes. The central maximum is much brighter than the other fringes. intensity The intensity of a single slit diffraction pattern displays some important features: distance from centre The pattern becomes more spread out if a narrower slit and larger wavelength of light are used. 24 of 34 © Boardworks Ltd 2009 Diffraction gratings A diffraction grating is a plate with many closely spaced parallel slits. It produces widely spaced interference patterns as a result of the superposition of waves from the many slits. The pattern from a diffraction grating consists of brighter and sharper fringes than the pattern from a double slit arrangement. The pattern is much clearer, which makes it possible to calculate the wavelength of the light more accurately. 25 of 34 © Boardworks Ltd 2009 The diffraction grating equation 26 of 34 © Boardworks Ltd 2009 The diffraction grating equation The diffraction grating equation: nλ = dsinθ Where d = slit spacing (equivalent to 1 / number of slits per m). Fractions of a degree are measured in minutes ('), where 1° = 60'. The maximum number of orders, n, can be found by substituting θ = 90° into the equation. Given that sin90° = 1: n = d / λ, rounded down the nearest whole number. 27 of 34 © Boardworks Ltd 2009 The spectrometer A spectrometer is an important application of diffraction gratings. It used to measure wavelengths of light very accurately. This has many applications, such as determining the chemical compositions of stars and hence showing the ‘red shift’ evidence for the expanding universe 28 of 34 © Boardworks Ltd 2009 Investigating diffraction gratings 29 of 34 © Boardworks Ltd 2009 Diffraction grating calculations 30 of 34 © Boardworks Ltd 2009 31 of 34 © Boardworks Ltd 2009 Glossary 32 of 34 © Boardworks Ltd 2009 What’s the keyword? 33 of 34 © Boardworks Ltd 2009 Multiple-choice quiz 34 of 34 © Boardworks Ltd 2009