Light: waves or particles?

In 1678, Christiaan Huygens wrote the book Traité de la lumiere ("Treatise on light"). He believed that light was made up of waves. He said that light could not be made up of particles because light from two beams do not bounce off each other. In 1672, Isaac Newton wrote the book Opticks. He believed that light was made up of red, yellow and blue particles which he called corpusles. Newton explained this by his "two prism experiment". The first prism broke light up into different colours. The second prism merged these colours back into white light.

During the 18th century, Newton's theory was given the most attention. In 1803, Thomas Young described the "double-slit experiment". In this experiment, light going through two narrow slits interferes with itself. This causes a pattern which shows that light is made up of waves. For the rest of the nineteenth century, the wave theory of light was given the most attention. In the 1860s, James Clerk Maxwell developed equations that described electromagnetic radiation as waves.

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Quantum theory of light

It turns out that electrons are dislodged by the photoelectric effect if light reaches a threshold frequency. Below this no electrons can be emitted from the metal. In 1905 Albert Einstein published a paper explaining the effect. Einstein proposed that a beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (photons), each with energy. Einstein said that the effect was due to a photon striking an electron. This demonstrated the particle nature of light.

Einstein also found that electromagnetic radiation with a long wavelength had no effect. Einstein said that this was because the "particles" did not have enough energy to disturb the electrons.

Einstein realized that the energy of each photon was related to the photon frequency by the Planck constant. This could be written mathematically as:

hc

hc

In 1921 Einstein received the Nobel Prize for linking the Planck constant to the photoelectric effect.

Colour of light emitting diodes

In the electric circuit shown on the right, the voltage drop across the light emitting diode (LED) depends on the material of the LED. For silicon diodes the drop is 0.6 V. However for LEDs it is between 1.8 V and 2.7 V. This information enables a user to calculate the Planck constant.

E = QeVL

Simple LED circuit that illustrates use of the Planck where constant. The colour of the light emitted depends on the voltage drop across the diode. The wavelength of Qe is the charge on one electron. VL is the voltage drop the light can be calculated using the Planck constant. across the LED.

When the electron decays back again, it emits one photon of light. The energy of the photon is given by the same equation used in the photoelectric effect. If these equations are combined, the wavelength of light and the voltage are related by

hc

VLQe

The table below can be calculated from this relationship.