PHY2054: Chapter 228Following Reflected and Refracted RaysÎRay 1 is the incident rayÎRay 2 is the reflected rayÎRay 3 is refracted into the luciteÎRay 4 is reflected in the luciteÎRay 5 is refracted as it enters the air from the lucitePHY2054: Chapter 229Chapter 22 QuizÎSnell’s Law describes: (a) Huygens’ construction (b) Magnification (c) Reflection (d) RefractionPHY2054: Chapter 2210Chapter 22 QuizÎFor refracted light rays, the angle of refraction: (a) is always equal to the incident angle (b) is always greater than the incident angle (c) is always less than the incident angle (d) is always equal to the critical angle (e) can be less than, greater than, or equal to angle of incidencePHY2054: Chapter 2211DispersionÎWhen light travels from one medium to another: Speed decreases Wavelength decreases Frequency constantÎRefractive index is variable Depends on λ n decreases as λ increasesnn()////nncf cnf nλλ===/nccn=PHY2054: Chapter 2212DispersionÎn decreases as λ increases For red light (λ = 700 nm) ⇒ n is smaller (less bending) For blue light (λ = 400 nm) ⇒ n is bigger (more bending) Spreading (“dispersion”) of colors due to refractionPHY2054: Chapter 2213Rainbows Formed by DispersionÎLight is refracted by spherical water droplets Sun must be behindyou (rainbow is circular, with sun on axis) Red light bent at a lesser angle (top of rainbow) Violet light bent at a greater angle (bottom of rainbow)PHY2054: Chapter 2214Understanding Refraction in Wave PictureÎConsider a row of soldiers slowing down in the mud Wave fronts “turn” due to change of speed in the medium Analysis based on Huygens principle Closer analysis yields Snell’s law with cn= c / nPHY2054: Chapter 2215Critical AngleÎθ < θcbeam partlyreflected & partlyrefracted at boundaryÎθ > θcbeam is entirelyreflected at the boundary Total internal reflection occurs only when n1> n221sin /cnnθ=90°21sin /cnnθ=PHY2054: Chapter 2216Critical Angles (to vacuum)24.4°2.42Diamond31.9°1.89Heaviest flint glass34.4°1.77Sapphire37.2°1.655Tooth enamel37.3°1.65Heavy flint glass40.4°1.544Quartz40.5°1.54Sodium chloride41.1°1.52Crown glass42.8°1.473Glycerine47.3°1.36Ethyl Alcohol48.6°1.333Water49.8°1.31Ice88.6°1.00029Air90.0°1VacuumCritical angleNSubstancesin 1/cnθ=PHY2054: Chapter 2217Intensity During Transition to Critical AngleÎTransition is actually very smooth Incident ray is reflected andrefracted As incident angle approaches critical angle, intensity of refracted ray goes to zero and reflected ray takes all the intensityPHY2054: Chapter 2218Fiber OpticsÎBased on total internal reflection Fiber has inner “core” of optically dense glass Outer “cladding” of less optically dense glassÎLight rays passed through core to make shallow reflections with surface ⇒ 100% reflection at each bouncePHY2054: Chapter 2219Fiber Optic ConstructionÎTrick is to make core absorb no light So light can be transmitted over long distances Requires ultra-pure glass (no impurities, variations)ÎManyfibers can be carried in a bundlePHY2054: Chapter 2220Construction of Fiber Optic CableSingle mode fiberPHY2054: Chapter 2221Medical applicationsÎVisual scoping of internal organs, arteries, joints, etc.ÎUses bundles of fibers to form an imagePHY2054: Chapter 2222Fiber Optic CablesPHY2054: Chapter 2223Communications ApplicationsÎOptical fibers can carry digital information Telephone, networks, InternetÎHigh frequency of light ⇒ high rate of information transfer “High bandwidth”, several Tb/s over single fiberÎSignals can travel ∼ 100 km before needing boost Not susceptible to electrical noiseÎReplacing copper wires for long distance communicationPHY2054: Chapter 2224Chapter 22 QuizÎTotal internal reflection (a) refers to light being reflected from a plane mirror (b) may occur when a fisherman looks at a fish in a lake (c) may occur when a fish looks at a fisherman on a lakePHY2054: Chapter 2225National Lambda Rail Fiber InfrastructureOnly NLR Backbone links shownwww.nlr.net/PHY2054: Chapter 2226Florida Lambda Rail (10 Florida
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