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Young’s Interference Experiment • In 1801, Thomas Young demonstrated the wave nature of light by showing that it produced interference effects • he measured the average of sunlight to be 570 nm • a single slit causes diffraction of sunlight to illuminate two slits S 1 and S 2 • each of these sends out circular waves which overlap and interfere

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Young’s Interference Experiment. In 1801, Thomas Young demonstrated the wave nature of light by showing that it produced interference effects he measured the average of sunlight to be 570 nm a single slit causes diffraction of sunlight to illuminate two slits S 1 and S 2 - PowerPoint PPT Presentation

Youngs Interference ExperimentIn 1801, Thomas Young demonstrated the wave nature of light by showing that it produced interference effectshe measured the average of sunlight to be 570 nma single slit causes diffraction of sunlight to illuminate two slits S1 and S2 each of these sends out circular waves which overlap and interfere

DiffractionHow do we know light is a wave?Waves undergo diffractionif a wave encounters an object that has an opening of dimensions similar to its , part of the wave will flare out through the openingcan be understood using Huygens argumenttrue for all waves e.g ripple tank

Water waves flare out when passing through opening of width aa

e.g. sound v=f=v/f = (340m/s)/1000Hz = .34 m a of door ~ 1 m => a~ 3 e.g. light ~ 500 nm = 5 x 10-7 m => need smaller openingtangent towaveletsa =4

Points of same phase add constructivelyVertical screen>>ad

CoherenceFor interference to occur, the phase difference between the two waves arriving at any point P must not depend on time.The waves passing through slits 1 and 2 are parts of the same wave and are said to be coherentlight from different parts of the sun is not coherentthe first slit in Youngs expt produces a coherent source of waves for the slits S1 and S2

Youngs Double SlitInterference pattern depends on of incident light and the separation d of the two slits S1 and S2 bright vertical rows or bands (fringes) appear on the screen separated by dark regions

Choose any point P on the screen located at an angle with respect to central axisWavelets from S1 and S2interfere at P. They are inphase when they enter theslits but travel different distances to P.Assume D>>d so that rays r1and r2 are approximately parallel

If L = 0, , 2 , 3 ,... then waves are in phase at P. ==> bright fringeIf L = /2, 3/2, 5/2 ,... then waves are out of phase at P. ==> dark fringeTriangle S1bS2 : S1b= L = d sin

Double SlitBright fringe: L = m d sin = m , m=0,1,2,

Dark fringe: L = (m+1/2) d sin = (m+1/2) , m=0,1,2,

use m to label the bright fringesm=0 is the bright fringe at =0central maximum

Bright Fringesm=1 d sin = =sin-1(/d)bright fringe above or below(left or right) of central maximum has waves with L = first order maxima

m=2 =sin-1(2/d)second order maxima0-11-22

Dark FringesFor m=0, =sin-1(/2d)first order minima

- Position of FringesBright fringes at d sin = m , m=0,1,2, sin = m (/d)y/D = tan e.g. =546 nm, D=55cm, d=.12mmhence /d ~ 4.6 x 10-3 Bright fringes for sin = m (/d)
- Separation of Bright FringesUsing the fact that sin ~ for