6.2 Two slit interference Coherence Two-Slit Interference Thin film Interference
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Transcript of 6.2 Two slit interference Coherence Two-Slit Interference Thin film Interference
6.2 Two slit interferenceCoherenceTwo-Slit InterferenceThin film Interference
Interference EffectsInterference is a general property of waves.A condition for interference is that the wave source is coherent.Interference between two waves gives characteristic interference patterns due to constructive and destructive interference.
CoherenceFor two waves to show interference they must have coherence. Two waves are coherent if one wave has a constant phase relation to the other coherent incoherentphaseshift
CoherenceLight fromtwo separatelight bulbs isIncoherentLight from a singlelight bulb passingthrough two slits iscoherent
path differencepath difference = =r2 r1In phaseCondition for constructive interferenceCondition for destructive interferencem = 0 + 1, + 2,.r1r2Superposition of wavesat A shows interferencedue to path differencesAOrder number m
Interference pattern of water wavesConstructiveDestructive
Interference pattern due totwo spherical wavesAmplitude on screen=0==-m=0m=1m=-1
Youngs two slit experimentThomas YoungDoes light show wave properties?
Path difference from the two slitspath differenceIn the limit L>>d, the rays are nearly parallel
Interference patternBright constructive interferenceDarkdestructive interferencem = 0, + 1, + 2, ............Central maximum
QuestionLight from a laser is passed through two slits a distance of 0.10 mm apart and is hits a screen 5 m away. The separation between the central maximum and the first bright interference fringe is 2.6 cm. Find the wavelength lf the light. for m= 1dsin=msin ~ for smallanglessolve for
Clicker Question 1In a two slit interference experiment, how does the separation between peaks in the interference pattern change if the distance between slits is increased?IncreaseDecreaseStays the sameIndeterminate
Clicker Question 2In a two slit interference experiment how does the distance between the peaks on the screen change if the wavelength of the light is increased?increasesdecreases stays the sameindeterminate
Thin film interferenceIn thin film interference the phase difference is due to reflection at either side of a thin film of transparent material.The phase difference is due to two factors:Path difference through the film (corrected for the change in speed of light in the material)Phase shift due to reflection at the interface
Phase shift due to reflectionn1 < n2phase shift=180oReflection with inversionphase shift = 180o
Phase shift due to reflectionReflection without inversionPhase shift = zeron1 > n2Phase shift = zero
Thin film InterferenceFor a thin film in air the phase difference due toreflection is 180oCondition for constructiveinterference=2t=The wavelength in the film is longer than in air.
Thin film InterferenceFor film in air the phase difference due toreflection is 180oCondition for destructiveinterference=2t=The wavelength in the film is longer than in air.
Soap film interference pattern
QuestionA vertical soap film displays a series of colored band due to reflected light. Find the thickness of the film at the position of the 5th green band (=550 nm, n =1.33)
Constructive Interference The 5th band has m=4 (the first is m=0)
Anti-reflective CoatingAnti-reflective coatings are usedto reduce reflections at the air-glassinterface.anti-reflectivecoatingno coating
Anti-reflective CoatingAnti-reflective coatings consists of a thin-layer of material with a refractive index in between that of air and glass. Destructive interference between light reflected at the two surfaces reduces the intensity of reflected light. n1=1.00 < n2 < n3Condition for destructive interference.tn1n2n3phase shift of 180o atboth interfaces.Total phase difference due to reflection is zero
QuestionAn anti-reflective coating of MgF2 (n=1.38) is used on a glass surface to reduce reflections. Find the minimum thickness of the coating that can be used for green light (=550 nm).For destructive interference Quarter wavelength (in coating) thickness minimumat m=0Solve for t
Optical compact discCd store information in a seriesof pits and bumps in the plastic.The information is read by a reflectedlaser beam. The intensity of the beam is changed byconstructive or destructive interferencedestructive interference