2.5.1 Collisions of Electrons with Atoms

• Ionisation of an atom is:

The removal, or addition, of an electron from, or to, an atom when given sufficient energy

• Excitation of electrons is:

When an electron is given enough energy to move up an energy level, but not enough to leave the atom

Fluorescent Tube

• Fluorescence occurs when an electron in an atomic orbital absorbs energy from an interaction with a photon or a collision with another atom
• Fluorescent tubes are partially evacuated glass tubes filled with low-pressure mercury vapour with a phosphor coating on the glass
• When an electric current is passed through the vapour, the electrons in mercury are excited and move to a higher energy level
• This high energy level state is unstable and so the electron moves back to its original state, or de-excites
• As it de-excites, the electron releases some of that energy in the form of a UV photon
• This UV light then excites the electrons in the phosphor coating
• As a result, visible light photons are released when the electrons return to their original energy state, which provides the fluorescent glow

Fluorescent tubes operate on the basis of excitation and de-excitation of electrons leading to the emission of visible light

• Photons are fundamental particles which make up all forms of electromagnetic radiation
• A photon is a massless “packet” or a “quantum” of electromagnetic energy
• What this means is that the energy is not transferred continuously, but as discrete packets of energy
• In other words, each photon carries a specific amount of energy, and transfers this energy all in one go, rather than supplying a consistent amount of energy

Calculating Photon Energy

• The energy of a photon can be calculated using the formula:

E = hf

• Using the wave equation, energy can also be equal to:

• Where:
  • E = energy of the photon (J)
  • h = Planck's constant (J s)
  • c = the speed of light (m s^-1)
  • f = frequency (Hz)
  • λ = wavelength (m)

• This equation tells us:
  • The higher the frequency of EM radiation, the higher the energy of the photon
  • The energy of a photon is inversely proportional to the wavelength
  • A long-wavelength photon of light has a lower energy than a shorter-wavelength photon