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Young's "Double Slit" Experiment
This experiment, performed by Thomas Young in 1801, was considered to give proof that light is a wave.  
Today we are more cautious and say that experiments of this type, involving diffraction and interference, prove that light has wave-like properties  
In other experiments, we find that light also exhibits particle like properties,(see the section on quantum theory for more detail).  
   
The diagram below (not to scale!) show the apparatus used.  
 
   
The light source is monochromatic (single colour) which means it gives out a narrow range of wavelengths.  
   
The distance, D, is about 2m.  
On the screen we see a series of bright and dark lines called interference fringes.  
   
For a constant interference pattern, we need two sources having a constant phase relation (two coherent sources).  
The single slit ensures that the two slits in the double slit are coherent sources.  
What is observed at a given point on the screen depends on the phase difference between waves from slit b and waves from slit a, when they arrive at that point.  
   
In the following explanation, we will assume that waves leave the two slits in phase, (this is not a necessary condition, it just makes the explanation easier).  
   
Point O is midway between the two slits so waves from the two slits will arrive at O in phase. .  
At O we see a bright fringe due to constructive interference.  
This fringe is the central maximum of the interference pattern.  
   
Point A is further from slit b than from slit a.  
The path difference is bA-aA  
If this is equal to λ/2 then destructive interference will occur and a dark fringe (a minimum) will be observed.  
   
Point B is even further from slit b than from slit a.  
The path difference is now bB-aB.  
If this is equal to then constructive interference will occur and a bright fringe will be observed.  
   
In general
Constructive interference will occur at points for which the path difference is equal to nλ
Destructive interference will occur at points for which the path difference is equal to (2n+1)λ/2
where n = 0, 1, 2, etc
 
   
The distance between adjacent bright (or dark) fringes is called the fringe spacing.  
The fringe spacing depends on  
- the distance between the two slits, d  
- the distance between the slits and the screen, D  
- the wavelength of the light, λ  
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