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UMD PHYS 375 - Reflection and Refraction of Light – Planar Surfaces

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Experiment 1 - Reflection and Refraction of Light 1Experiment 1Reflection and Refractionof Light – Planar Surfaces1 IntroductionIn this series of experiments we shall study the phenomena of reflec-tion and refraction of light from planar surfaces. We will study Snell’slaw and the law of reflection experimentally by investigating refractionat interfaces, refraction through a prism and total internal reflection.Throughout this experiment, we will be using the geometrical opticsapproximation to analyze the behavior of light.In this experiment you will use a laser!Be careful not to shine it into your or your neighbor’s eye!Always know where your beam is directed!2 Background - see Pedrotti3, Sections 2-1 to 2-5 and3-3When an optical ray encounters a boundary between two transparentmedia, usually part of the light is reflected from the boundary andpart enters the second medium. Each medium is characterized by adimensionless parameter called the index of refraction, the ratio of thespeed of light in vacuum to the speed in the medium, designated by n1and n2. A light ray is incident in medium 1 and strikes the interfacebetween the two media at an angle α with respect to the normal to thesurface (see Figure 1). The transmitted ray propagates in a differentdirection than that of the incident ray. This change in direction iscalled refraction. The angles of the reflected and refracted rays aredetermined by the following laws:1. The angle of incidence, α, is equal to the angle of reflection.Experiment 1 - Reflection and Refraction of Light 2ααβMedium 1 Medium 2n1 n2Incident RayReflected RayRefracted RayNormalFigure 1: Reflection and refraction of light from an interface.2. The angles of incidenc e and refraction are related byn1sin α = n2sin β, (1)known as Snell’s law.3. The reflected and refracted rays lie in a plane defined by the inci-dent ray and the normal to the interface at the point of incidence.3 ExperimentExperimental Hint: To determine angles with respect to the theglass surfaces, use retro-reflection - adjust the surface so that the re-flected light exactly retraces the laser beam. Then you know the beamwas normal to the surface (the Law of Reflection again ).3.1 Reflection and Refraction of a Glass BlockUsing a laser beam as a source for your rays, measure appropriate anglesto determine the index of refraction of a glass block relative to air.A way to estimate your uncertainties in measuring angles is is to testyour ability to verify the Law of Reflection.3.2 Total Internal Reflection With a Semi-Circular BlockIf light is propagating from medium with n1to n2and n1> n2,Snell’s Law no longer has a solution for incidence angles greater thanthe critical angle, where the angle of refraction is 90◦. For larger angles,no light is transmitted - it is all totally internally reflected. The criticalangle is given bysin θtir=nontir(2)where ntir> n0.Experiment 1 - Reflection and Refraction of Light 3Figure 2: Semi-circular block showing the path of a light ray at theonset of total internal reflection.Use a measurement of the critical angle to determine the index ofrefraction of a s emi-circular block of glass (see Figure 2). Why usea semi-circular block of glass, and not the block from the previousexperiment?3.3 Minimum Deviation Angle in a PrismIn Fig. 3, there is an illustration of a prism and the path of a light raythrough the prism. The minimum value of δ (as a function of rotationof the prism) is δmand is related to the minimum incident angle, θm,by the equationδm= 2θm− A. (3)Furthermore, one can show that the index of refraction of the prism isgiven bynang=sin θmsin(A/2). (4)Thus, to determine nangof the prism, one must determine θmand A.Find the minimum deviation angle and the index of refraction of theprism. If you plot δ vs. θ you will find it varies quadratically, and youare tasked to find the θ value where δ is smallest. Think about howthis will influence your ability to determine the minimum angle and theeffect on your uncertainties .Experiment 1 - Reflection and Refraction of Light 4ABscreenx!"Figure 3: A light ray is shown passing through a prism; it is thenprojected onto a viewing screen. The angles δmand θmare as


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UMD PHYS 375 - Reflection and Refraction of Light – Planar Surfaces

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