Particle Physics

Discrete energy and radioactivity

Emission and absorption spectrum

Energy levels: The orbits of electrons around the nucleus.

When a potential difference is applied to a tube of a gaseous element, the gas will begin to glow. When an electron is excited it absorbs energy to move to a higher energy level and it as it becomes de-excited it emits energy to fall to the lower energy level. This emitted light is what causes the glow.

emission spectrum

Passing this light through a slit allows us to separate it into its different wavelengths. This is called the Emission spectra. The emission spectrum shows coloured lines at the different discrete energies 

absorption spectrum

The absorption spectrum is the continuous spectrum of light with the emission spectrum subtracted. the electrons will absorb the same wavelengths which they emitted during de-excitation. 

Discrete energy and discrete energy levels

Looking at the emission spectra, we see distinct lines and not a continuous spectrum of colour. The reason for this is that electron energy is quantized into discrete energies.

Discrete: Countable. Opposite of continuous.

Discrete energy: Energy of photons being emitted or absorbed can only take certain discrete values.

Discrete energy levels: Energy levels, are discrete with specific values of energy which all electrons require to be in that level.


These discrete packets of energy are called photons. Photons are both waves and particles, thus they can be discrete as well as having a wavelength. To calculate the energy of a photon, we can use plank's equation:

Wavelength form Velocity form
\(E=h\frac{c}{\lambda}\) \(E=h\nu\)
  • h=plank's constant
  • c = speed of light
  • \(\lambda\) = wavelength
  • h = plank's constant
  • v = velocity photon

Transitions between energy levels

Photoelectric effect




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