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Electromagnetic Radiation & Quantum Phenomena (Photoelectric Effect…
Electromagnetic Radiation & Quantum Phenomena
Photoelectric Effect
The photoelectric effect occurs when light above a certain frequency (the threshold frequency) is shone on metals like zinc, this causes electrons to escape from the zinc. The escaping electrons are called photoelectrons.
the frequency of the light needed to reach a particular minimum value (depending on the metal) for photoelectrons to start escaping the metal
the maximum kinetic energy of the photoelectrons depended on the frequency of the light not the intensity of the light
The energy of a photon of light = hf and the work function (f)is the minimum energy required to remove an electron from the surface of the material. So we can see from the equation above that if the light does not have a big enough frequency (f) so that the photon has enough energy to overcome the work function (f) then no photoelectrons will be emitted.
Excitation
The electron volt (eV) is an amount of energy.
It is the amount of energy an electron would gain if it was accelerated through a potential difference of 1 volt.
1 eV = 1.6 x 10⁻¹⁹ joules (J) of energy
In atoms electrons orbit the nucleus
There are particular allowed orbits where electrons can exist without emitting energy. Electrons can pass between these energy levels. When electrons are given enough energy to move to higher energy levels they are in an excited state, this is called excitation.
If an electron gets enough energy to remove the electron to infinity this is called ionisation.
Line Spectra
Emission Spectra
A diffraction grating and a spectrometer can be used to look at the emission spectrum from a light source.
If all possible wavelengths of light are present it would look like a continuous spectrum of colours.
However hot gases emit only particular characteristic colours of light.
Each line in the emission spectrum corresponds to an electron moving from a higher energy level to a lower energy level. To do this it emits photon of light the energy of the photon of light is equal to the difference in the energy of the two energy levels.
Absorption Spectra
When white light passes through a gas the gas absorbs particular wavelengths of light. This effect can be seen in light from the sun which initially seems like a conscious spectrum but an closer inspection it can be seen to contain dark lines.
Wave-particle Duality
Sometimes light behaves like a wave and sometimes light behaves like a particle.
Diffraction - can be explained by considering light to be a wave.
Photoelectric effect – can be explained by considering light to be a particle.
Therefore we use the concept of wave-particle duality when thinking about light.
Momentum
The momentum of a particle can be calculated by multiplying it’s mass in kilograms (kg) by it’s velocity in metres per secons (m s⁻¹). Momentum is measured in kilogram metres per second (kg ms⁻¹)
momentum = mv
De Broglie wavelength
De Broglie suggested that all particles not just light exhibit wave-particle duality and therefore it would be possible to calculate the wavelength of particles
(the De Brogiel wavelength).
