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Interference of Light



double slit apparatus

screen display

double slit theory




Conditions for interference


1. The waves from light sources must be coherent with each other. This means that they must be of the same frequency, with a constant phase difference between them.


2. The amplitude (maximum displacement) of interfering waves must have the same magnitude. Slight variations produce lack of contrast in the interference pattern.




Young's Double Slit Experiment - Apparatus



Young's Slits apparatus



It is important to realise that the diagram is not to scale.


Typically the distance (D) between the double slits and the screen is ~ 0.2 m (20 cm).

The distance (a) between the double slits is ~ 10-3m (1mm).

The preferred monochromatic light source is a sodium lamp.



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Young's Double Slit Experiment - Display



Young's Fringes display with sodium light



The image above is taken from the central maximum area of a display.


You will notice some dimming in the image from the centre travelling outwards. This is because the regular light-dark bands are superimposed on the light pattern from the single slit.


The intensity pattern is in effect a combination of both the single-slit diffraction pattern and the double slit interference pattern.


In other words, t he amplitude of the diffraction pattern modulates the interference pattern.


The diffraction pattern acts like an 'envelope' containing the interference pattern.




modulated single slit  diffraction pattern with interference pattern




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Young's Double Slit Experiment - theory


The separation (y) of bright/dark fringes can be calculated using simple trigonometry and algebra.


Consider two bright fringes at C and D.


For the fringe at C, the method is to find the path difference between the two rays S1C and S2C . This is then equated to an exact number of wavelengths n.


A similar expression is found for the fringe at D, but for the number of wavelengths n+1 .

The two expressions are then combined to exclude n .




Young's Slits theory behaviour




With reference to triangle CAS2 , using Pythagoras' Theorem:


Young's Slits theory - equation #2a            


substituting for AC and S2A in terms of xC , a and D,



Young's Slits theory - equation #1            (i



also, with reference to triangle CBS1



Young's Slits - theory - equation #1a       


       Young's Slits theory - equation #2            (ii




Subtracting equation (ii from equation (i ,




Young's Slits theory - equation #3


 Young's Slits theory - equation #4




Using 'the difference of two squares' to expand the LHS,



Young's Slits theory - equation #5





Young's Slits theory - equation #6




The path difference S2C - S1C is therefore given by:



Young's Slits theory - equation #7




In reality, a ~ 10-3m and D ~ 0.2 m .


So the length a is much smaller than D (approx 1/200 th D).


The two rays S2C and S1C are roughly horizontal and approximate to D.




Young's Slits theory - equation #8


cancelling the 2's,

Young's Slits theory - equation #9




For a bright fringe at point C the path difference S2C - S1C must be a whole number (n) of wavelengths (λ).



Young's Slits theory - equation #10


Rearranging to make xC the subject,


Young's Slits theory - #11



Similarly, for the next bright fringe at D, when the path difference is one wavelength longer, that is equal to (n+1) wavelengths ,


Young's Slits theory - equation #12


hence the fringe separation   xD - xC  is given by,



Young's Slits theory - equation #13

Young's Slits theory - equation #14



assigning the fringe separation the letter y ,


Young's Slits theory - equation #15


or with wavelength λ the subject,


Young's Slits theory - equation #16






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