The Particle Nature of EM Radiation (Edexcel A Level Physics)

The Particle Nature of Light

• In classical wave theory, electromagnetic (EM) radiation is assumed to behave as a wave
• This is demonstrated by the fact EM radiation exhibits phenomena such as diffraction and interference
• However, experiments from the last century, such as the photoelectric effect and atomic line spectra, can only be explained if EM radiation is assumed to behave as particles

Evidence for the Particle Nature of Light

• The best evidence for the particle nature of light comes from the photoelectric effect
• This is demonstrated using the Gold-leaf Electroscope

Observations of the Gold Leaf Experiment

• The explanation for these observations supports the theory of light as a particle, specifically a discrete packet (or photon) of energy
  • Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
  • Using a higher frequency light source does not change how quickly the gold leaf falls
  • Using a filament light source causes no change in the gold leaf’s position
  • Using a positively charged plate causes no change in the gold leaf’s position
  • Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal
• Each of the observations is explained below

Typical set-up of the gold leaf electroscope experiment

Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly

• Placing the UV source closer to the plate increases the intensity incident on the surface of the metal
  • Increasing the intensity, or brightness, of the incident radiation increases the number of photoelectrons emitted per second
  • Therefore, the gold leaf loses negative charge more rapidly

Using a higher frequency light source does not change how quickly the gold leaf falls

• The maximum kinetic energy of the emitted electrons increases with the frequency of the incident radiation
  • In the case of the photoelectric effect, energy and frequency are independent of the intensity of the radiation
  • So, the intensity of the incident radiation affects how quickly the gold leaf falls, not the frequency

Using a filament light source causes no change in the gold leaf’s position

• If the incident frequency is below a certain threshold frequency, no electrons are emitted, no matter the intensity of the radiation
  • A filament light source has a frequency below the threshold frequency of the metal, so, no photoelectrons are released

Using a positively charged plate causes no change in the gold leaf’s position

• If the plate is positively charged, that means there is an excess of positive charge on the surface of the metal plate
  • Electrons are negatively charged, so they will not be emitted unless they are on the surface of the metal
  • Any electrons emitted will be attracted back by positive charges on the surface of the metal

Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal

• A single photon interacts with a single electron
  • If the energy of the photon is equal to the work function of the metal, photoelectrons will be released instantaneously