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.

**Momentum**

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})