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Quantum physics interference and diffraction provide evidence for the wave model
electromagnetic (EM) radiation is assumed to behave as a wave which is demonstrated by
the fact EM radiation exhibits diffraction and interference
However photoelectric effect and atomic line spectra can be explained if EM radiation
behaves as particles (particle model).
Photons
Definition: packet of energy of electromagnetic radiation
What this means is that energy is transferred as discrete packets of energy rather than
being transferred continuously
Photons are fundamental particles which make up all forms of electromagnetic radiation
E = hf (can also be written as E = hc/)
Where E = energy of photon (J)
h = Planck’s constant (6.63 x 10−34 )
f = frequency (Hz)
= wavelength (m)
c = speed of light (3 x 108 ¿
This equation tells us
E f (high frequency radiation means high frequency photons)
E 1/
Gamma photons (EM radiation from nuclei) are the most energetic
Electron volt
Used to express very small energies because quantum energies tend to be much smaller
than a joule
One electron volt (eV) is the energy gained by an electron travelling through a potential
difference of 1 volt
1 eV = 1.6 x 10−19 J
When a charged particle is accelerated through a p.d. , it gains K.E. which is given by
1 2
eV = mv
2
rearranging to find speed of electron gives √ 2 eV /m
(this equation applies to all charged particles)
E = VQ
Photoelectric effect
Definition: emission of electron from the surface of a conductor when electromagnetic
radiation is incident on it
The electrons removed in this manner are called photoelectrons
The photoelectric effect provides important evidence that light is carried in discrete
packets because each electron absorbs only 1 electron so only the frequencies of light
above a threshold frequency will emit a photoelectron.
Threshold frequency: minimum frequency required to release electron from the surface
of a metal
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