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IntroductionTo explain how light behaves, we can think of lighttravelling as rays.A ray travels in a straight line. It will change directionif:• It is reflected (when it strikes a surface)• It is refracted (when it passes from one material toanother).We can also think of light as wavesLaws of reflectionThe laws of reflection tells us where a ray will go whenit is reflected.The normal is the line at 90o to the reflecting surfaceat the point where the incident ray strikes it2
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3normalincident ray reflected rayi rLaw 1: angle of incidence = angle of reflection, i = r(angles measured from the normal to the ray)Law 2: incident ray, reflected ray and the normal are allin the same plane.These laws not only apply for flat surfaces but also forcurved and rough surfacesnormali r ir
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Refraction: when it happensLight travels fastest in a vacuum. It travels more slowlyin other media.When light changes speed (because it travels from onemedium to another), it is refracted.• If a ray enters a medium head-on (angle of incidence i= 0), it travels straight on.• If a ray enters a medium obliquely, it bendsLaws of refractionAs with reflection, angles are measured from the normalto the ray.Law 1: Snell’s law explains how the angles of incidenceand refraction are related.Law 2: incident ray, refracted ray and the normal are allin the same plane.4
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5normalr ri iNote that r is now the angle of refraction, not reflectionRefractive index nThe refractive index, n of a medium relates the speedof light in the medium to the speed of light in free space(vacuum)Refractive index, n =speed of light in free spacespeed of light in mediumn =c0cmedium
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Refractive index nIn a medium of refractive index 2, light travels at halfits speed in free space (vacuum). Some values of n areworth remembering:n0 = 1 (by definition)nair = 1.00 (to 2 decimal places)nwater = 1.33nglass ~ 1.5 (depending on the composition of glass)Snell’s lawFor a ray passing from air into a medium of refractiveindex n, the angle of incidence i and the angle ofrefraction r are related by:6n =sin isin r
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Snell’s lawWhen a ray passes from one medium to another, therefractive index can be calculated using the equations:7n =cicr=nrniExampleA ray of light travels from glass (ni = 1.5) into water(nr = 1.33) with an angle of incidence i of 30o. Calculatethe angle of refraction r.Step 1: calculate the relative refractive index from thevalues for the two materials:n =nrni=1.331.5= 0.887
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8Step 2: Substitute values into the Snell’s lawequation, rearrange and solve:n = sin isin rso 0.887 =sin 30osin rsin r = sin 30o0.887= 0.504 so r = 34oQuestions:1. Does a ray speed up or slow down when it enters amore dense medium?2. If the angle between the incident ray and thereflective surface of a mirror is 35o, what are theangles of incidence and refraction?3. A ray of light, travelling through air, strikes a glasssurface with an angle of incidence of 40o. Therefractive index of glass is 1.47. draw a diagram toshow the situation. Calculate the angle of refraction.
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