AQA A Level Physics

Revision Notes

2.5.1 Collisions of Electrons with Atoms

Ionisation & Excitation

  • 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 Tube, downloadable AS & A Level Physics revision notes

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

The Photon Model

  • 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:

2.5.1 Photon Energy Equation

  • 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 higher energy than a shorter-wavelength photon

Worked Example

Light of wavelength 490 nm is incident normally on a surface, as shown in the diagram.

The power of the light is 3.6 mW. The light is completely absorbed by the surface.

Calculate the number of photons incident on the surface in 2.0 s.

2.5.1 The Photon Model Worked Example

Exam Tip

Make sure you learn the definition for a photon: discrete quantity / packet / quantum of electromagnetic energy are all acceptable definitions.

The values of Planck’s constant and the speed of light will always be available on the datasheet, however, it helps to memorise them to speed up calculation questions!

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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