WAVE PARTICLE DUALITY
For centuries, physicists all over the world were leading a heated debate about the nature of light and for many years there were two opinion groups among them. Supporters of the first group believed that light was a wave, while members of the other group believed that electromagnetic radiation had a particle nature. However, quantum mechanics showed that neither of the groups was completely right and that the real answer to this question is much stranger and much more complicated than any of the contemporary thinkers could have ever imagined.
YOUNG’S EXPERIMENT
Young’s experiment, often referred to as the double-slit experiment, is a relatively simple experiment. It was used at the beginning of the 19th century to prove that light exhibits wave properties. This experiment exploits two specific properties of waves:
1. If a wave reaches a small opening, it bends. This phenomenon is called diffraction. The size of the opening has to be comparable to the wave’s wavelength for diffraction to occur.
Scheme of diffraction – a wave bends after passing through an opening between two walls
2. When two waves encounter, they do not collide but strengthen or weaken each other depending on what the displacement (“height”) of both waves is. This phenomenon is called interference. For example, when two waves with opposite displacements meet (i.e. a crest of one wave meets a trough of another wave), they cancel out. If interfering waves weaken each other, we are talking about destructive interference. The opposite of destructive interference is constructive interference (waves strengthen each other).
Scheme of destructive interference – two waves weaken each other
Scheme of constructive interference – two waves strengthen each other
In the double-slit experiment, two slits, which are very close to each other, are used. Light passes through both slits and spreads to the medium behind the opening thanks to diffraction. Due to small distance between the slits, the waves from the first slit meet the waves from the second slit and interference occurs. If we situate a plate detecting the position of individual beams of light that strike it, a specific pattern is created, the so-called interference pattern, which consists of light and dark stripes. Light stripes on the plate are located in places where – 10 – constructive interference of light waves occurs (waves strengthen one another, thereby increasing the intensity of light incident on these places), dark stripes are caused by destructive interference (waves weaken one another, thereby decreasing the light intensity). If light did not exhibit wave properties, interference pattern would not be created.
Scheme of Young’s experiment – light goes through two slits and diffraction occurs. Then, light from one slit interferes with light from the other slit and interference pattern is created on the plate. Behind the detection plate, there is a graph showing the amount of light incident on certain places of the plate. The graph shows that between the slits, constructive interference occurs. In areas directly behind the slits, destructive interference prevails.
Young’s experiment is a simple experiment demonstrating the wave nature of electromagnetic radiation. The original version of this experiment is not related to quantum mechanics, but using its modifications, we can easily prove some of the strange phenomena of the microworld, as we shall see in the following chapters.