Diffraction is a specific form of electromagnetic (EM) interference.
The classic example of interference is ** Young's experiment**.

A light source is first restricted by a single **pinhole**,
P_{0}, and then by two equidistant pinholes, P_{1} and P_{2}, and the result observed on a screen. While Young
used pinholes, today we normally use ** slits** to make the interference
pattern easier to see.

Several things are observable in the pattern on the screen obtained using a white light source:

- the central vertical line is white, indicating no colour (wavelength) separation there
- a single colour (eg, red) forms a series of equally spaced vertical lines on the screen
- Colours are separated so that longer wavelengths are further from the centre
- the intensity decreased away from the centre, becomes very low, then increases slightly, before dropping off again.

The overall change in intensity from the centre to the sides of the screen is consistent with a single slit diffraction pattern.

The colour separation and series of vertical coloured lines
is Young's **interference**. The intensity from the source can
be reduced so that individual photons pass through slit P_{0} and when sufficient number have been measured the same pattern is
seen on the screen. The only conceivable interpretation for this
phenomenon is that each photon from the source which passes through
the first slit P_{0} must then pass through both slits P_{1} and P_{2} before being detected at a point **x** on the
screen. A particle could not do this but an EM disturbance could.

A photon entering pinhole P_{0} has its spatial location
severely restricted. From **Heisenberg's uncertainty principle** this means its momentum is very much broadened. It therefore leaves
the slit, still travelling at the speed of light, but with no preferred
direction. It therefore spreads out and adopts nearly spherical
symmetry. The "spread-out" disturbance is then restricted
in two locations simultaneously by pinholes P_{1} and P_{2} so that again the two disturbances exit with no preferred direction.

The two disturbances then reach the screen, but the screen is not
capable of responding to part of a photon disturbance. They will
therefore be detected as a **whole photon** at random somewhere
on the screen but with a probability at any point equal to the strength
of the disturbance at that point.

A photon is an undulating disturbance with a characteristic wavelength.
We do not need to know its exact shape, merely that it repeats regularly
once each wavelength. Any fixed point on this undulation can be
labelled as a '**wavefront**'. If the wavefronts of the two disturbances
exiting pinholes P_{1} and P_{2} arrive at point **x** at the same time they will create a combined disturbance
twice as great as their individual disturbances. There is therefore
a strong likelihood that such a strong disturbance would be detected
on the screen.

This combined disturbance will repeat as we look along the screen
whenever the **difference in path length** (r_{1}-r_{2})
corresponds to one wavelength. Colours with longer wavelengths (eg,
red) will therefore take a longer distance to repeat than shorter
wavelengths (eg, blue).

Because of the **finite** size of the pinholes, the momentum
is not completely broadened and so the exiting wavefronts will not
be perfectly spherical. There is therefore a limit to how far the
slits P_{1} and P_{2} can be placed apart.