1 reflection and refraction


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1 reflection and refraction

  1. 1. 11 Reflection and Refraction1
  2. 2. 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
  3. 3. 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
  4. 4. 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
  5. 5. 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
  6. 6. 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
  7. 7. 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
  8. 8. 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.