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Wave Particle Duality and Quantum Theory

Physics (Year 12)


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Dev Lohar

Wave-particle duality of light

In your earlier school life, you were taught that light is a form of wave. However, experiments conducted in 19th century proved that light behaved as a particle. So is light a wave or a particle?

It is now known that light behaves both as a wave, and as a particle (the particles are known as photons). This is known as the wave-particle duality of light. Here, we will explore the wave nature of light and the evidence that supports it.

Double slit experiment

In 1801, an English scientist by the name of Thomas Young performed the Double Slit experiment which supported the wave model of light. Below is the diagram commonly used to describe this experiment.


The experiment went as follows:

  • Monochromatic light is directed through a single slit and then through a double slit. Monochromatic light means light of a single frequency.

  • When the light passes through the first single slit, it diffracts and circular waves are formed which pass through the double slit where they diffract again.

  • The 2 waves will interference with each other constructively (when waves are in phase) and destructively (when waves are out of phase).

  • The constructive interference results in bright spots on the screen and the destructive interference results in dark spots on the screen.

If light behaved as a particle, then we should only observe 2 bright spots on the screen.


The distance between adjacent bright spots can be calculated using the following equation:

Path difference

Path difference is defined as the difference in distance travelled by 2 waves (each from the 2 slits) to a point on the screen. Path difference is measured in wavelengths instead of metres.


Constructive interference occurs when the path difference is in whole number multiples of wavelength (eg. 0, 1, 2, 3, …). Destructive interference occurs when the path difference is in odd number of half wavelengths (eg. 1/2, 3/2, 5/2, …).



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