Electromagnetic radiation

  • A form of energy emitted and absorbed by accelerating charge
  • EMR has both electric and magnetic field components, which are perpendicular to each other and to the direction of propagation
  • EMR is self-propagating and requires no medium, since the changing electric field generates a changing magnetic field, which in turn generates an electric field
  • This creates an infinite loop


  • Planck and Einstein discovered that energy in EMR could only exist in discrete packets (quanta) of energy
  • These were called photons
  • They had no mass and travels at c

Energy of a photon is given by E = hf

Where h is Planck’s constant, f = frequency and e = energy in joules

The intensity of light is directly proportional to the number of photons it emits.

Electromagnetic spectrum

The spectrum is the range of all possible frequencies (and wavelengths) of EMR

All EMR travel at the speed of light, but not with the same energy levels

Blue light has more energy than red light, as it is at a higher frequency and shorter wavelength.

Wave-particle duality

  • Light behaves both like a particle and a wave
  • Light diffracts, refracts, and reflects like a wave
  • Photoelectric effect proves that light acts like a particle

Young’s double slit experiment

  • Young illuminated two narrow slits with a light source.
  • The resulting pattern on the screen behind the slits showed an interference pattern, which proves that light acts like a wave.

Photoelectric effect

  • When light of a certain frequency shines on a metallic surface, electrons are emitted
  • If light does not have a high enough frequency, no electrons are emitted
  • This effect can only be explained if light were a particle

Wave-particle duality of light

Property Can be explained in terms of waves Can be explained in terms of particles
Reflection Yes Yes
Refraction Yes Yes
Interference Yes No
Polarisation Yes No
Photoelectric Effect No Yes
Diffraction Yes No





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