Photons
Physics: IB Diploma Higher Level, topic 5.1.3
Photon Model of Electromagnetic Radiation
We can think of electromagnetic radiation (light) as little packets of energy. We call these packets photons.
Frequency and energy
- The energy of a photon is directly proportional to the frequency of the radiation.
- The letter 'h' is the Planck constant.
- It is a fundamental constant of nature and is equal to 6.63×10-34 Js.
Energy of different photons
- Gamma photons have the highest frequency and so carry the most energy.
- Radio photons have the lowest frequency and so carry the least energy.
The Electron Volt
When dealing with energy levels of individual atoms, the SI unit of energy (the Joule, J) is far too big. So we need to define a more appropriate unit of energy, the electron volt, eV.
Definition of electron volt
- The electron volt is defined as the energy given to a fundamental charge, e, accelerated through a potential difference of 1 Volt.
Conversion
- To convert between J and eV, simply multiply or divide by the charge of the electron, e = 1.6×10-19.
- 1 eV = 1.6 ×10-19 J
Example - hydrogen
- Let's say the energy of an electron in a hydrogen atom is 13.6 eV.
- To find this in Joules, use the equation shown previously:
- 13.6 eV = 13.6 × 1.6 ×10-19 J
- 13.6 eV = 2.18 ×10-18 J (3 s.f.)
Test yourself on Photons
What is the symbol used for Planck's constant?
What is the definition of the electron volt?
What is 1.9 eV in Joules?
What is 25 eV in Joules?
What is 0.07 J in eV?