# Physics A level revision resource: The effects of interference

In 1672, Isaac Newton published his theory that light was made up of particles of matter, but in 1678 Christiaan Huygens claimed to disprove Newton with evidence that light was actually a wave. It wasn't until the discovery of quantum physics around 100 years ago that we began to resolve exactly what light is.

## So is light a wave or a particle?

In 1801 Thomas Young devised a very simple experiment to determine just that. Using a visible light source, he first made the light coherent by firstly passing it through as single slit and then through a double slit. From this he successfully demonstrated the wave-nature of light - since the interference pattern produced could be accounted for by the superposition principle. The waves emerging from either slit recombines either constructively (creating peaks of light) or destructively (creating troughs).

Using only a laser and a double slit we can reproduce Young’s double slit experiment, since the laser is already a coherent light source.

Remember that precision and accuracy are different things. In a laboratory setting, accuracy is the difference between your measurement and the true value, whereas precision describes the variation in your measurement in making repeat readings of the same value. For accurate measurements, it is important to use the piece of apparatus with the smallest scale - a micrometer will give you a value closer to the true length than a metre rule. However, if you attempted to measure a distance of tens or hundreds of centimetres with a micrometer then your tool would be imprecise - every time you measured this distance you would get a very different reading. This means that there is usually a trade-off between accuracy and precision when trying to get the best measurements.

Very accurate tools like micrometers and vernier calipers are useful over small distances, but they have different actions and may be more or less precise depending on their design. For example, the micrometer that Maggie is using in the picture is very good at measuring the size of objects that fit within its jaws, like the diameter of a wire. However, they are much less precise if used for measuring the distance between something like the interference peaks in the Young's slit experiment - you should think about how accurately you record your reading accordingly.

All the variables in this experiment are distances and therefore the sources for error are down to our ability to measure these distances both precisely and accurately. To ensure that your results are as reliable as they can be, it should be clear that the choice of apparatus is appropriate to the required range of measurement. When measuring the wavelength of light, repeating the experiment several times for a variety of distances between slits and screen, and a range of slit separations should give you a more reliable set of results. Remember to also take repeat readings and work out an average. Taking repeat measurements is important even for very experienced experimental researchers since not only does it improve the precision and accuracy of the result, but it also demonstrates that the experiment is replicable.

## What can we learn from Young's experiment?

Young’s experiment demonstrates the wave-nature of light since the interference pattern produced cannot easily be explained if light were only particle-like. We now know that the pattern produced by the superposition of waves of light can be used to either determine the wavelength of light when the lengths/distances are known, or to determine the size of small separations when the wavelength is known. The key equation in this calculation is, λ = ax/D, where λ is the wavelength of light, a is the slit separation, x is the fringe separation and D is the distance between the slit and the screen, all measured in meters.

The results of Young’s slit experiment still underpins much of the physics research carried out in universities today that uses light and other kinds of electromagnetic radiation. Recently, the detection of gravitational waves use our understanding of light to detect the interference in a laser beam when a gravitational wave passes through a detector. The laser beams in the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors travel 4km distances but a passing gravitational wave would cause a disturbance much smaller than the size of an atom! So in this case, making sure that the distance measurement is both precise and accurate is quite an undertaking.

The original Young’s slit experiment supported Huygens' original idea of light as a wave, however it can be taken a step further to show the wave/particle duality of light. If you repeat this experiment with a light source that releases single photons of light, the same interference pattern gradually builds up, even though detectors show that only a single photon is travelling through one or other of the slits at any given time. This requires the photon to be in a state of quantum superposition, and the double slit experiment is a directly observable example of this phenomenon - you might be familiar with the concept of superposition from the thought experiment of Erwin Schrödinger usually referred to as 'Schrödinger's cat'.