PowerPoint PresentationSlide 2Electromagnetic Waves are Transverse WavesSlide 4Photons, Power, and Energy Contained in LightSlide 6The Particle Nature of LightDual Nature of LightGeometric Optics – Using a Ray ApproximationRay ApproximationSlide 11Ray Approximation -- Reflection of LightSpecular ReflectionDiffuse ReflectionLaw of ReflectionRefraction of LightSlide 17Following the Reflected and Refracted RaysThe Index of RefractionSlide 20Refraction Details, 1Refraction Details, 2Slide 23DispersionDispersion: Variation of Index of Refraction with WavelengthRefraction in a PrismThe RainbowSlide 28Observing the RainbowSlide 30Slide 31Slide 32Physics 213General PhysicsLecture 1502Last Meeting: Electromagnetic Waves, Maxwell Equations Today: Reflection and Refraction of LightElectromagnetic Waves are Transverse WavesThe and fields are perpendicular to each otherBoth fields are perpendicular to the direction of motionTherefore, em waves are transverse wavesErBr4Photons, Power, and Energy Contained in Light Intensity (I) is average power per unit areaLight comprises photons with each one having E=hfh is Plank’s constant=6.63x10-34JsPhotons are quantum mechanical particles that move at speed of light.o o oAv erage power per unit areaE B E c BIc 2 2max max max max2 2 2 6The Particle Nature of Light“Particles” of light are called photonsEach photon has a particular energyE = h ƒh is Planck’s constanth = 6.63 x 10-34 J sEncompasses both natures of lightInteracts like a particleHas a given frequency like a waveDual Nature of LightExperiments can be devised that will display either the wave nature or the particle nature of lightIn some experiments light acts as a wave and in others it acts as a particleNature prevents testing both qualities at the same timeGeometric Optics – Using a Ray ApproximationLight travels in a straight-line path in a homogeneous medium until it encounters a boundary between two different mediaThe ray approximation is used to represent beams of lightA ray of light is an imaginary line drawn along the direction of travel of the light beamsRay ApproximationA wave front is a surface passing through points of a wave that have the same phase and amplitudeThe rays, corresponding to the direction of the wave motion, are perpendicular to the wave fronts11Ray Approximation -- Reflection of LightA ray of light, the incident ray, travels in a mediumWhen it encounters a boundary with a second medium, part of the incident ray is reflected back into the first mediumThis means it is directed backward into the first mediumSpecular ReflectionSpecular reflection is reflection from a smooth surfaceThe reflected rays are parallel to each otherThis is the type of reflection from a smooth metal surface or a mirror.Diffuse ReflectionDiffuse reflection is reflection from a rough surfaceThe reflected rays travel in a variety of directionsRough metallic surfaces show diffuse reflection – cannot see image on their surface.Law of ReflectionThe normal is a line perpendicular to the surfaceIt is at the point where the incident ray strikes the surfaceThe incident ray makes an angle of θ1 with the normalThe reflected ray makes an angle of θ1’ with the normalThe angle of reflection is equal to the angle of incidenceθ1= θ1’Refraction of LightWhen a ray of light traveling through a transparent medium encounters a boundary leading into another transparent medium, part of the ray is reflected and part of the ray enters the second mediumThe ray that enters the second medium is bent at the boundaryThis bending of the ray is called refractionRefraction of LightWhen a ray of light hits a transparent medium part of the ray is reflected and part of the ray enters the second mediumThe ray that enters the second medium is bent at the boundaryThis bending of the ray is called refractionThe incident ray, the reflected ray, the refracted ray, and the normal all lie on the same planeThe angle of refraction, θ2, depends on the properties of the medium1212sinsinvvFollowing the Reflected and Refracted RaysRay is the incident rayRay is the reflected rayRay is refracted into the transparent mediumRay is internally reflected in the transparent mediumRay is refracted as it enters the air from the transparent mediumThe Index of RefractionWhen light passes from one medium to another, it is refracted because the speed of light is different in the two mediaThe index of refraction, n, of a medium can be definedSome values of nFor a vacuum, n = 1For other media, n > 1n is a unitless ratioAs the value of n increases, the speed of the wave decreasesspeed ofl ight in a vacuum cnspeed ofl ight in a medium v 201212sinsinvvRefraction Details, 1Light may refract into a material where its speed is lowerThe angle of refraction is less than the angle of incidenceThe ray bends toward the normalRefraction Details, 2Light may refract into a material where its speed is higherThe angle of refraction is greater than the angle of incidenceThe ray bends away from the normal23Physical Interpretation of RefractionDispersionThe index of refraction in anything except a vacuum depends on the wavelength of the lightThis dependence of n on λ is called dispersionSnell’s Law indicates that the angle of refraction made when light enters a material depends on the wavelength of the lightDispersion: Variation of Index of Refraction with Wavelength The index of refraction for materials commonly decreases with increasing wavelengthViolet light then refracts more than red light when passing from air into a materialSnell’s Law indicates that the angle of refraction made when light enters a material depends on the wavelength of the lightRefraction in a PrismThe amount the ray is bent away from its original direction is called the angle of deviation, δSince all the colors have different angles of deviation, they will spread out into a spectrumViolet deviates the mostRed deviates the leastThe RainbowA ray of light strikes a drop of water in the atmosphereIt undergoes both reflection and refractionFirst refraction at the front of the drop, then reflection at back, and a final refraction at exitViolet light will deviate the mostRed light will deviate the least28Observing the RainbowIf a raindrop high in the sky is observed, the red ray is seenA drop lower in the sky would direct violet light to the observerThe other colors of the spectra lie in between the red and the
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