(3)Light and Society

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  • Created on: 25-03-14 13:29
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Light and Society.
A question which has puzzled scientists for many years is, is light a particle or a wave?
Scientists disagreed with these ideas for a very long time. Scientists like Newton believed
that the only explanation was that light behaved as a particle. However Hooke and
Huygens believed in the wave theory. Thomas Young showed through his double slit
experiment that light could only be explained in terms of waves. However light does tend
to behave like a wave sometimes, but also like a particle other times. Light is therefore
referred to as a wavicle - or a wave-particle. Sometimes it behaves like one or the other,
and other times it behaves like both.
Max Planck tried to work out how a blackbody emits radiation. A black body can emit and
absorb all wavelengths of light, and in our universe, stars are natural black bodies.
However, he was ahead of his time, in terms of knowledge, since most of his work was
not understood when he published it. His work was later continued by Einstein. His
advanced knowledge meant he had to assume some ideas which were not known about
at the time, such as he had to assume that energy could behave in discrete amounts, not
just continuous amounts. Through this theoretical work Planck came up with a new
constant, known as Planck's constant - which is 6.63x10^-34Js. He calculated from this
that black bodies behaved like oscillators, which lost and absorbed energy in equal
In 1905 Einstein continues Planck's work. Einstein the developed of quantise radiation.
These are like energy `packets' which are called photons. Planck's constant can be used
in an equation, which allows scientists to work out the energy of photons. The equation is
E = hf or Energy of photon = Planck's constant x frequency. Planck's constant is usually
symbolised with the letter h.
Furthermore, if you were to shine a light on a surface, then that surface would absorb the
energy. If you increase the amount or `intensity' of that light, then you increase the energy
absorbed. This idea is known as radiation flux, and the basic principle is that a higher
radiation flux will be caused by a higher intensity of light over a smaller area. This seems
straightforward, but radiation flux is based on an equation containing power not energy.
Therefore radiation flux is the power per unit area. So the energy transferred to the
surface per second per unit area in more complex terms. This can be derived as the
equation F = P/A. Based on this equation, we can calculate the units of radiation flux. If
Power is in watts and area is m x m which is m2, then the units for radiation flux are Wm-2.
Another idea Einstein came up with is the photoelectric effect. Metals are made from ions
and a `sea' of free moving delocalised electrons. If energy is supplied to a metal, like
electricity the electrons can be made to move. In a similar way, giving energy to a metal
can make electrons escape from their metallic structure. So if you take a gold leaf
electroscope, the leaf would generally fall slowly, as some electrons are lost to the air,
but they have little energy so electrons are not generally lost. However energy such as UV
light, can excite the electrons, and so cause them to escape from the metal more quickly.
This causes the leaf to fall more quickly, due to the charge leaking away more quickly.
How quickly the leaf falls is dependent on the intensity of the light, and its distance from
the metal plate on the electroscope. Visible light does not have the same effect- in fact
is has no effect on the leaf.

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Before Einstein carried out this work, scientists already knew that UV light had this effect
on metals. Electrons that escape from the metal are called photoelectrons. As mentioned
before visible light has no effect, and so no photoelectrons are made when visible light is
shone on the metal. This is because every surface has a threshold frequency, below
which no photoelectrons are made.…read more


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