What is Light – a Particle or a Wave?

Light, particularly light beyond the visible spectrum, is probably the most important tool for modern analytical instrumentation and material characterization. This is why I would like to briefly discuss the historical development of light theories and our current understanding of it.

While the character of light was already given a lot of thought in ancient times, the real scholarly dispute began in the late 17th century. Christiaan Huygens, the Dutch astronomer and physicist, opined that light was a wave. He based this on a wave’s propagation in water and was able to solve essential optical questions in an analogous fashion, thus building better telescopes, for example. His main opponent was Isaac Newton, the British natural scientist, philosopher and most important advocate of the corpuscular theory of light. This theory describes light as a stream of particles whose different mass represents their colors. At that time no experiments were known that could have solved this dispute, so the more prominent scientist came out on top: Isaac Newton swayed popular belief in the 18th century with his corpuscular theory.

Light waves

At the dawn of the 19th century the British physician and physicist Thomas Young performed experiments that showed that phenomena such as Newton’s rings, which defied the corpuscular theory (!), could be explained by means of the wave theory. Newton’s rings appear in an oil puddle on a wet street, for example, making it fluoresce in all kinds of colors (picture 1). These can be explained by means of interference, the fact that waves can amplify or quench each other. Based on this and other scientists’ work, the wave theory grew more popular and the corpuscular theory was put on the back burner.

Oil Puddle
Picture 1: Oil puddle (©iStockphoto.com/nickpo)

Light particles

In the year 1900 Max Planck published his theory about the black body radiation. This can be thought of as the radiation coming out of a small opening of a well isolated oven glowing inside. The visible radiation changes its color from red tones to a stark white when the temperature of the oven is increased from several 100 °C to more than 3000 °C. This radiation not only contains visible light but also extends to the infrared and ultraviolet light. To describe this correctly, Planck had to assume that the radiation energy in the oven’s inner chamber was quantized. This means that the radiation energy cannot go to any possible level, but only to multiples of a quantum, the smallest possible energy portion. He also had to assume that the energy of the quantum depended on the light color or wavelength. Planck was in doubt about his own theory for many years; however, he was awarded the Noble prize for physics for it in 1918 and today is celebrated as the founder of the quantum theory.

The only person who immediately took to Planck’s theory was Albert Einstein. He published a paper based on it in 1905, in which he describes the photoelectric effect. Electrons are kicked out from a metal surface by light and can then be detected as an electric current. The light color may now determine whether a current flows. There is the case, for example, that there is no current flow under red light, no matter how strong it is, while the current flows under yellow, green and blue light (picture 2). Einstein saw the cause of this effect in the fact that light itself was quantized, meaning it consisted of minimal energy portions. These present particle characteristics and shoot electrons from the metal if they have enough energy to do so. However, their energy only depends on the color, ergo the wavelength of the light. These quanta of light were later called photons. In 1909, in a speech at a congress of German natural scientists and physicians, Einstein already pointed out that some effects could only be explained by means of both light models combined – the wave as well as the particle model.

Albert Einstein - Lichtelektrischer Effekt
Picture 2: Photoelectric effect

Wave and particle

In the scope of quantum physics, a field developed in the 20th century, we now refer to the wave-particle duality of light. This concept states that light can neither be viewed as just a wave nor just a particle, and that the application determines which characteristic dominates over the other. In this, light goes far beyond our limited powers of imagination. When asked what a photon actually was, Einstein is supposed to have given this caustic response at the end of his life: “Today every Tom, Dick and Harry thinks they know what a photon is – but they don’t.” This is why experiments are still being conducted to better understand light. Particularly in recent decades, technologies have been developed that allow for experiments with single photons and lead to spectacular new applications.

“Today every Tom, Dick and Harry thinks they know what a photon is – but they don’t.”

Albert Einstein

Light plays an integral role with many Anton Paar products: In SAXSpace, for example, samples’ structures are measured in the nanometer range by shooting X-ray photons at them, which have a much higher energy than visible light. Part of these photons is scattered by the sample’s electrons and creates a scattering pattern on a detector, providing information on the sample’s inner structure. MCP polarimeters measure by which angle the oscillation plane of polarized laser light is rotated by a sample. This helps differentiate variants of pharmaceuticals that show the same chemical behavior but affect patients differently. The Alcolyzer on the other hand employs infrared light to determine the alcohol concentration in beverages. A certain part of the infrared light that shines through the sample is turned into mechanical oscillation of the alcohol molecule and is therefore lacking from the light behind the sample. This missing part is a direct measure of the sample’s alcohol concentration.

Finally I would like to recommend a humorous English-language talk by Alain Aspect, the well-known French experimental physicist, who provides an interesting insight into historical and current light research:

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