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 wavelike 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 bAaA 

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 bBaB. 

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 
