IB DP Physics: HL复习笔记12.1.1 Photons

• 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 discovering the photoelectric effect and atomic line spectra, can only be explained if the EM radiation is thought of as behaving as particles

• These experiments have formed the basis of quantum theory, which will be explored in detail in this section

Photons

• Photons are fundamental particles which make up all forms of electromagnetic radiation
• A photon is defined as

A massless ''packet'' or a ''quantum'' of electromagnetic energy

A wave packet (photon)

• This means is that the energy of a photon 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
• The energy of a photon can be calculated using the formula:Where:

• • E = energy of the photon (J)
  • h = Planck's constant (J s)
  • c = the speed of light (m s-1)
  • f = frequency in Hertz (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 lower energy than a shorter-wavelength photon

Step 1: Write down the known quantities

• • Wavelength, λ = 490 nm = 490 × 10−9 m
  • Power, P = 3.6 mW = 3.6 × 10−3 W
  • Time, t = 2.0 s

Step 2: Write the equations for wave speed and photon energyPhoton Momentum

• Einstein showed that a photon travelling in a vacuum has momentum, despite it having no mass
• The momentum (p) of a photon is related to its energy (E) by the equation:

• Where
  • c = the speed of light
  • p = the momentum of the photon (kg m s-1)
  • E = the energy of the photon (J)

Step 1: Write down the known quantities

• • Power, P = 5.0 mW = 5.0 × 10−3 W
  • Frequency, f = 5.6 × 1014 Hz
  • Cross-sectional area, A = 8.0 × 10−6 m2

Step 2: Write the equations for photon energy and momentumExam Tip