For hundreds of years, there was a lot of ambiguity surrounding the nature of light – was it made from waves or particles? There was evidence for both possibilities so which one was it.
Light acting as a wave:
In 1803, Thomas Young carried out his famous double slit experiment – the conclusions drawn from this experiment showed that light acted like a wave. Young’s double slit experiment, involved splitting the light from a candle in order to show an interference pattern.
The set up:
He first set up a screen with one singular slit in order to make the light source monochromatic (as white light consists of many different wavelengths). This singular slit refracted all the red light (since red light diffracts more than blue light), therefore Young was left with only one wavelength of blue to be refracted. After the singular slit, he placed a screen with two slits that refracted the blue light (the slits were roughly the same size as the wavelength in order to get the cleanest and most distinct interference pattern possible).
The conclusions:
(The picture below (figure 1) shows the visible results from the double slit experiment)
As you can see, the light has been split and has dark gaps between each fringe. This is due to each wave constructively interfering with each other at the bright points and deconstructively interfering with each other at the dark fringes (a complete explanation for why this occurs contains a few more details than that). However, the very act of interference and interference patterns is a property exclusively displayed by waves meaning that light must therefore act as a wave.
Light acting as a particle:
In 1887 Heinrich Rudolf Hertz carried out the photoelectric effect – the conclusions drawn from this experiment showed that light acted like a particle. The photo electric effect involved shining light (of any EM wave) onto a metal surface. As a result, free electrons in the metal would absorb the lights energy and be released from the metal (these electrons were called photoelectrons).
The conclusions:
After various trials, Hertz found that the photoelectrons would not be released if the light was below a certain frequency value (known as the threshold frequency). This observation could not be explained by the wave theory of light (as according to the wave theory of light, the photoelectrons should be released regardless of the frequency of the wave), however this observation could be explained using Einstein’s photon model of light. This involved imagining light as being made of particles rather than waves. Since a photons energy is dependent on frequency, this explains the threshold frequency, concluding that light therefore must act as a particle.
So after these two really important experiments, we are left with two definite conclusions:
1. Light acts a wave
2. Light acts a particle
But how is this possible? How can light act as a both a wave and a particle?
Wave-particle duality theory:
Well, in 1924 Louis de Broglie came up with a solution: Wave-particle duality theory. He conducted an experiment, which involved firing a stream of electrons at a crystal lattice. This caused the electrons to be diffracted like a beam of light – it created a diffraction pattern of concentric circles. Since interference and diffraction is inherently a property of waves, this is showing that particles have wave-like properties. De Broglie suggested that all matter gave of wavelengths corresponding to their velocities (wave-particle duality).
This therefore solved the question of what light is made of – it’s made of both particles and waves.
References:
Figure 1:
Bright, Richard. “The Double Slit Experiment Explained (?).” Interalia Magazine, 5 Dec. 2014, www.interaliamag.org/blog/the-double-slit-experiment-explained.
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