Particle property of waves
In our quest to understand the physical world before us, let us move a step further. Even an elementary student can say with conviction that the physical world before us in not what it appears us to be. Barring space, each and everything in this world is actually composed of certain fundamental particles viz. electrons, protons etc. Different arrangements of these particles give rise to different types of matter having so different characteristics.
Our physical world has its roots in microscopic world. The material cause of this world is elementary particles. The question, therefore, really is, “what is a particle?”.
In our normal understanding, a particle is an object which has a mass and which occupies some space. Intuitively, it implies an object having unique position in space at a particular time and also, some physical properties viz. mass etc. can be attributed to it. It is found that as we go microscopic, all these attributes disappear. However, the premise of a unique position at a particular time remains (to the best of my understanding). It becomes rather impossible, as we go microscopic, to imagine the particle in the terms of physical attributes like mass.
Another concept in Classical Physics is that of waves. The moment one says ‘wave’, the first picture one gets in mind is that of water ripples. In traditional Physics, the mechanics of particles and optics of waves are treated as different disciplines. However, it is found, that in microscopic world, the difference between particles and waves blur.
In order to understand as to how the difference blurs, it is important to understand as to what a wave is. It is basically an oscillation that travels accompanied with energy. This oscillation can be of anything. By oscillation, I mean that at every point of the span of wave, a physical or mathematical parameter will have variable value with respect to time. In order to understand this, let us take the example of water ripples. The variable physical parameter in this case is the height of water particle. In case of sound waves, the variable parameter is pressure. In case of electro-magnetic waves, it is electric field and magnetic field with a phase difference of ninety degrees. The distance between the crests is known as wavelength, λ. The animated representation of sound waves can be seen here. In case of electro-magnetic waves, you can see this.
It has been found that electro-magnetic waves travel at speed of light. These waves can be either visible or invisible, depending only on one parameter, λ. Since these waves (or for that matter, any wave) carry energy, we can feel or ascertain their presence even though they are not visible. They exhibit the common phenomenon pertaining to waves such as diffraction, interference etc.
Having thus understood the basic concepts pertaining to particle and wave, it seems amazing as how the difference can blur. Wave for e.g. is spread over a span whereas particle is localized. However, the physical world reveals strange things to us. Light is established as an electro-magnetic wave. But the experiment demonstrated Photo-electric effect by which it is found that the energy of electro-magnetic wave is not spread over wave fronts but is rather quantized in packets of energy. The distinguishing feature is again the ubiquitous λ. Thus, energy of a wave with wavelength λ, is not uniformly distributed over the span of wave but is in small packets and is equal to hc/λ. These small packets, in the case of light, are called photons. It can be imagined as a particle. Strangely, the energy of the particle depends on the λ of wave.
To understand it in practical terms, let us say that a person lights a candle. Light in the form of electro-magnetic waves starts and reaches till the point where its intensity dies to zero. In that span, variable electric fields and magnetic fields at a phase difference of ninety degrees get established. Along with this, however, infinite photons having quantized energy also start moving within that span. One can try very hard to visualize it. To me, it seems quite difficult.
Einstein said, “All these fifty years of conscious brooding have brought me no nearer to the answer to question, ‘what are light quanta?’.
It is clear now, after this discussion, that what appears to us to be is not necessarily what it is. The simple innocuous light which fills this entire space before our eyes cannot clearly said to be what it is. It exhibits the properties of two different concepts.