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  • A-Level Physics

Quantum Physics

In 1900, Max Planck discovered that EM energy travelled in little packets rather than as a continuous wave (like charge, EM radiation is quantised - hence the name), suggesting EM radiation was, in fact, particulate. These little packets of energy are called photons. The photon model is used to explain how EM radiation interacts with matter, while the wave model explains its propagation through space.


The energy of a photon is directly proportional to its frequency:

E = hf energy = Planck′s constant × frequency

Planck’s Constant is 6.63 E−34


This can be combined with the wave equation for the speed of light in a vacuum:

E = hc/λ energy = Planck′s constant × 3.00 E8 / wavelength

Rearranging this shows that photon energy is inversely proportional to the wavelength.


Electron Volts

Because photon energy is so small (a red photon has an energy of 3.00 E-19 J), the electron volt, eV, is used. It is defined as the energy transferred to/from an electron when it moves through a p.d. of 1 Volt:

1 eV = 1.60 E−19 J
1 J = 6.25 E18 eV

Investigating Planck's Constant

Planck’s constant can be investigated by seeing at exactly what p.d. an LED goes on – the threshold voltage. This, multiplied by the elementary charge, gives the wave energy, and since we know the frequency of the light, we can deduce Planck’s Constant. Repeating this for a number of different colours will give a more accurate result.




The Photoelectric Effect

When UV radiation is shone onto certain metals, like zinc, electrons are released from the surface of the metal. These electrons are called photoelectrons, and a gold leaf electroscope can be used to demonstrate this occurring:

Gold Leaf electroscope diagram, photoelectric effect diagram, photoelectric effect, photoelectron emission. EngineeringNotes
  • The electroscope is given a negative charge, causing the gold leaf to stand erect (as like charges repel)

  • UV light is shone onto the zinc plate

  • If enough electrons are photoelectrons, the electroscope loses its charge and the gold leaf falls down

There are three key observations from the photoelectric effect:

  1. Photoelectrons are only emitted if the UV radiation shining onto the zinc plate is above a certain frequency - this is called the threshold frequency. If the higher the frequency is above the threshold, the greater the kinetic energy of the emitted photoelectron.

  2. If the radiation is above the threshold frequency, photoelectron emission is immediate and instantaneous - suggesting that it is brought about by one particle of radiation hitting the surface (a photon).