4.5.1 - Photons Flashcards
What is a quantum?
A small discrete unit of energy.
What is a photon?
A quantum associated with electromagnetic radiation.
What is the quantum model?
- 1900, Max Planck
- Work on the electromagnetic radiation emitted by hot objects, such as stars or a red-hot glowing lump of metal.
- Made him realise that the radiated energy could not have continuous values.
- Planck hypothesised that the total radiated energy had to be emitted in packets, with each packet or quantum having a definite, fixed amount of energy.
Describe the photon model of electromagnetic radiation.
- 1905, Einstein
- Used quantum idea to show that atoms absorb and emit light in individual packets of energy.
- Idea that light sometimes behaves in ways that can only be explained by thinking of it as consisting of particles.
- Louis de Broglie
- Showed that light could behave as both a particle and a wave.
- Quanta of light or other electromagnetic energy became known as photons.
What equation can be used to calculate the energy of a single photon?
E = hf
photon energy (J) = Planck’s constant (6.626 * 10^-34 Js) * frequency of the radiation (Hz)
What is the alternative equation for calculating the energy of photons?
E = hc / λ
photon energy (J) = Planck’s constant (6.626 * 10^-34 Js) * speed of light (3 * 10^8 ms^-1) / wavelength (Hz)
Define the term ‘electron volt’.
The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt.
Why is the electron volt used?
Since the energies associated with individual photons are very low, when expressed in joules, these very small numbers require an answer expressed in standard form with a large negative power. So a more convenient unit often used by scientists is called the electron volt.
How do you convert from electron volts (eV) to joules (J)?
Multiply by 1.602 * 10^-19.
How do you convert from joules (J) to electron volts (eV)?
Divide by 1.602 * 10^-19.
