Nature of Light

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Physics Paper 2 (The nature of light)
The waveparticle duality theory states that anything that has particle properties can also have properties of waves. Quantum mechanics regards light as both streams of particles and as a wave.
Particles of light are called photons.
Energy is absorbed and radiated in discrete amounts rather than a continuous stream of energy. The energy is quantised and consists of particle packets of energy.
The energy of a photon and the frequency of the equivalent radiation wave are directly proportional to Planck's constant:
E = hf
h = 6.63x10-34
How intense light shines on something is known as radiation flux. This is a measure of the amount of energy landing on a unit area in a unit time. The units are Wm2.
For a full understanding of the behaviour of light, it must be interpreted as a waveicle. It behaves sometimes like a classical wave, and sometimes like a particle.
If energy is supplied to metals in the form of heat or light, the electrons can be liberated. Electrons liberated by exciting them with light are called photoelectrons. This happens in the photoelectron effect.
Threshold frequency:
The classic wave model for energy does not fit this effect for a number of reasons.
Only light above a certain frequency (threshold frequency) would liberate electrons. The classic model would suggest that any frequency would be sufficient as it would be continuously absorbed until it had
enough energy. This is not true as it is a 1 for 1 exchange: 1 photon=1 electron liberated (and any left over energy is kinetic). Photons have energy which is directly related to the frequency and therefore those with
low frequency have too little energy.
Immediate emission:
Provided the frequency is sufficient, electrons are emitted immediately even if the radiation has low intensity. The classic model would suggest that electrons would take time to absorb the necessary energy and the
time would be greater for less intense radiation. In the waveivle model, a single photon is enough to liberate one electron (if the threshold frequency is reached).
Intensity and number of emissions:
The intensity of the light has effect on the number of electrons being liberated but not on their kinetic energy. The classic model suggests that the intensity would affect both these variables. Greater intensity means
more photons per second, so more electrons are released.
The work done liberating an electron is known as the work function.
= hf 0
Where f0 is the threshold frequency.

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The total energy supplied is the sum of the energy required to liberate the electrons and the kinetic energy they have after their release.
E = + Ek
1 mv2
hf = hf 0 + 2
When considering the energy of subatomic particles, it is more convenient to use the electronvolt (eV) as the unit measure for energy. 1eV is transferred when 1 electron travels through a potential difference of 1
1eV = 1.…read more

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In lasers passing photons stimulate other electrons to fall down a level and emit the characteristic, continuous beam of light.
Photovoltaic cells produce an electric current because a photon can excite an electron to a level in which it can flow as a conduction electron. The composition of the material will depend upon the energy of the
photon needed.…read more


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