Answer: When it behaves like a wave!
In the first quarter of the
twentieth century physicists began to realise that particle did not always behave like particles –
they could behave like waves. They called this Wave-Particle Duality.
This theory
suggests that there is no really basic distinction between a particle and a wave. The differences
that we observe arise simply from the particular experiment that we are doing at the
time.
Some important steps in the development of the wave-particle theory were as
follows:
1923 discovery of the Compton effect
1923 de Broglie's matter-wave
theory
1927 the diffraction of electrons and the Uncertainty Principle of
Heisenberg
An interesting idea concerning waves and particles was put forward by G.I.
Taylor in 1924. He reasoned that if light only behaved like a particle, then in an interference
experiment such as Young's slits it would be possible to reduce the intensity to a point where
only one photon was passing through the apparatus at a particular time and you would therefore
expect there to be no interference - the photon could not interfere with itself. This does not
happen, however - the interference pattern is always visible although very faint, suggesting
some kind of wave property of the photon.
Diffraction appears to be some kind of
statistical behaviour of each individual photon, and not a reaction between photons. If we
attempt to determine which slit a given photon has passed through we destroy the diffraction
pattern, because we have altered the nature of the experiment.