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Wave-particle Duality

Sometimes light behaves like a wave and sometimes light behaves like a particle.

  • Diffraction – can be explained by considering light to be a wave.
  • Photoelectric effect – can be explained by considering light to be a particle.

Therefore we use the concept of wave-particle duality when thinking about light.


The momentum of a particle can be calculated by multiplying it’s mass in kilograms (kg) by it’s velocity in metres per secons (m s-1). Momentum is measured in kilogram metres per second ( kg m s-1 )

momentum = mv

De Broglie wavelength

De Broglie suggested that all particles not just light exhibit wave-particle duality and therefore it would be possible to calculate the wavelength of particles (the De Brogiel wavelength).

  • l = the de Broglie wavlength of the particle in metres (m)
  • h = the Plank constant 6.63 x 10-34 J s
  • m = mass of the particle in kilograms (kg)
  • v = velocity of the particle in metres per second (m s-1)
de broglie wavelength