Physicsdept\davew\3rdform\sow\resources\wave-particle.doc
Is Light a Wave Phenomenon or Particle Phenomenon?
From the beginning of time people have had different theories on what light is made up of.
Ancient thinkers, for example such as Aristotle, thought that we saw the world by sending
something out of our eye and it then being reflected back. In the "Dark-Ages", the Arabs thought
light was a ‘beam’ or ‘ray’ and on this idea they managed to describe reflection and refraction at
the beginning of this millennium (well before Western people understood it).
However, Arabic science ‘stalled’ and the Europeans caught up and by the end of the
seventeenth century, there were two conflicting views of light. Was it a wave phenomenon or
particle phenomenon? Isaac Newton performed experiments and decided that light was made of
particles and therefore the Corpuscular theory was correct and the wave theory was wrong. Due
to Newton’s powerful reputation in the fields of Mathematics, Physics and Chemistry, people
trusted him and his evidence for the Corpuscular theory. He had two main sources of evidence
to prove his theory of particular light.
1. An opaque object in the path of a beam of light casts a sharp shadow. Particles
travelling in straight lines would be expected to do just this. On the other hand, if light
were a wave motion, it ought to bend (diffract) round an obstacle in its path, as other
wave motions were known to do. This would not produce sharp shadows.
2. Light can travel through a vacuum. Whilst there was obviously no difficulty with the
idea of particles travelling thorough a vacuum, there was with waves (at this time no
known wave could).
Newton believed reflection happened due to a Reflecting force that pushed the particles away.
To describe refraction, Newton showed that as a cannonball hit water (a denser substance than
air), it slowed down and bent away from the normal and when light hit the denser material it bent
towards the normal thus showing the light went faster in denser materials. As before, he stated
that there was a downward force operating in the region between the air and water that
accelerated the particles of light in the vertical component.
At the same time Huygens, (1629-1695) tried to explain light as a wave. Although he had
explained reflection and refraction he could not explain colours at all so his theory was not
accepted.
Physicsdept\davew\3rdform\sow\resources\wave-particle.doc
However, Newton had difficulty explaining diffraction and interference of light with his
Corpuscular theory of light. He tried to explain the phenomenon by developing complex ideas of
oscillating forces on the particles of light. This was the first admission that observation implied
that light was associated with some wave-like properties.
This theory stayed popular for about 100 years. The problem with the Corpuscular theory was
that it did not explain properly how colour, diffraction and interference happened. Around 1800,
Thomas Young looked at the diffraction of a single colour of light from two small but close holes.
The light produces alternating areas of bright and dull intensity, interference patterns ‘fringes’.
This phenomenon could only be explained by the wave theory of light and was very easy to
show with water waves. Young then used Huygen’s wave theory of light waves, and proved
what Huygen could not. That is that coloured light had its own wavelength. Young later went on
to show how white light could be split into different colours. This was the real link that proved
Newton’s Corpuscular theory of light was wrong.
So was that it? Is light a wave? In 1900 Max Planck was dealing with a problem of black-body
radiation. The problem was that the wave theory predicts that small wavelengths should be
accompanied by a large intensity whilst with black-body radiation the wavelength is small, the
intensity and energy output is small. Planck created an equation that worked with all the
experiments he and others devised. This equation described the energy of light as changing in
discrete lumps or packets, quanta, of energy instead of the steady and continuous flow
expected of a wave phenomenon. The problem was these discrete packets of energy behaved
like particles. Planck’s theory of packets of energy interested Einstein who was working on a
problem called the photoelectric effect.
The photoelectric effect is when light hits a metal surface and electrons are given off. According
to the wave theory increasing the intensity of that light will give off more electrons. The problem
is that this does not happen. To increase the number of electrons coming off the metal the
frequency of the light must be raised. Einstein used the theory of discrete packets of energy,
which he called photons to solve this. Einstein deduced with this that the each electron needs a
certain minimum amount of energy to escape from the metal surface, known as the work
function. The escape of an electron is caused by the absorption of one photon with enough
energy. If the photon frequency is below the work function the election cannot escape.
Therefore throwing more photons (brighter or higher intensity light) at the metal will not help if
none of the photons has high enough energy. That is why the brightness of light does not
influence the energy gained by individual elections.
So what then is light? Is it a wave or particles in the form of packets of energy? Light does show
all the behaviour of waves with reflection, refraction and diffraction but it does show particle
properties with black-body radiation and the photoelectric effect. This is known as the waveparticle duality. Sometimes it is best to use the model of light as a wave and sometimes it is
best to use the model of light as a particle to obtain the results for your experiment. These
models are not alternatives, but two complementary parts of a fuller model.
Reference
http://www.liverpoolcollege.org.uk/Science%20Magazine/light.htm