Energy and matter, before Einstein were considered to be two completely different things, that are governed by two mutually independant laws of conservation. However, Albert Einstein's famous equation E = mc2 (where E is the energy, mass is the mass of the body and c is the velocity of light), changes those views and revealed that mass and energy are actually inter-related and can be converted into each other. However, this discovery marked the beginning of a problem, that has been confusing scientists till date - if energy can be converted into mass, and vice-verca, then in what form is energy transferred - as waves or as particles?

So far, we know of both types of experiments, those which are explined by the wave-theory of light, and those that require the corpuscular theory ofr explanation. There is however, no known experiment in which light behaves both as a wave as well as a particle. Results obtained depend upon the type of experiment performed.

Let us consider the differences between a wave and a particle.

Particles are discrete. Their energy is concentrated into a finite space, one which has definite boundaries and homogenous contents that are the same at any point within the particle. Particles exist at specific locations, represented by a set of as many co-ordinates as there are dimensions, each co-ordinate representing one. Particles can never exist in more than one places at the same time. Their motion is governed by the the laws of kinetics and mechanics. Simple laws like the law of conservation of energy and the law of conservation of momentum govern how particles interact during collision. These essential basic laws are a part of Newtonian mechanics and were first published in his principia mathematica in 1687.

Waves, on the other hand cannot be considered a finite entity. Since by definition, the energy of waves varies both in dosplacement as well as time, their energy cannot be considered to exist in a single place.

A wave, unlike a particle can propagate, in an area of space until it exists in all locations and at all times. A pure sine wave is to represent a wave in nature mathematically. Such a wave has no beginning or end, a fixed period after which it repeats itself. Hence, we just need to analyse a part or a phase of the wave and obtain a value for its velocity within this area. Some fundamental features that govern the nature of a wave are the frequency (f), wave number (k) and wavelength (8).

Light is a deformation of electric (E) and magnetic (B) fields in an area of space, which oscillate in planes that are perpendicular to each other.

These two known ways in which the transmission of energy occurs - waves and particles seem to be completely different from each other. However, both of them perfectly seem to befit the title - 'component of energy.

Many supporters of Newton believed in a clockwork universe, one that was governed by absolute laws independant of the position of the observer. The exact position and momentum of every particle could be found, and hence the history of the universe was known and the future could be determined. However, wave-particle duality model dealt a fatal blow to these ideas.

Werner Heisenberg, in his famous Uncertainity Principle, explained that it was impossible to determine the position and momentum (or velocity) of a particle at the same instant. This can be explained by deBroglie's equation: wavelength = h / momentum ( h is the Planck's constant )

As one knows, a pure sine wave has a fixed wavelength, and so the particle represented by the wave has a fixed momentum according to the above equation, which thus can be determined. But, since a pure sine wave is an infinite entity, spreading out it both directions, we are completely uncertain as to where on the sine wave is the particle. It can be anywhere on the wave!

It is also known that adding together several waveswith different wavelengths produces a resultant wave, that is somewhat localized, due to interference. The more waves that are added together, the more localized the resultant wave is, till finally it is reduced to one point, which can be the position of the particle. However, with so many wavelengths, which one woud give the momentum of the particle when deBroglie's equation is used. Hence, we are completely uncertain about its momenum!

The Uncertainty Principle can also be explained thus: To try to measure the position of a particle, it has to be observed. But it can only be seen by the light reflected by it. Hence to determine the position of a particle, we have to shine some light on it. But, when such a tiny particle receives energy in the form of a light wave, its momentum changes. Hence, we can never be sure about its momentum.

The uncertainty principle dissapointed many scientists that were attepting to look for the absolute laws that Newton had proposed. How can one go on to predict what will happen in the future, if he is not even sure what the present looks like? Heinsenberg principle applies to many fields, even in today's electronics and the human brain. The power of free-will is just another example. What we decide out of a number of choices is uncertain, and cannot be determined by our past thoughts or our environment.

The experiments of the nineteenth century blurred the classical definitions of particles and waves. We can no longer assume that waves behave consistently with the theories that have been formulated for them. These theories can infact, sometimes be better applied to particles. Waves can, actually exhibit some particle-like behaivour.

The first phenomena that alarmed believers of the standard wave theory was thermal radiation. By analysing the radiant intensity of electro-magnetic radiation across the spectrum at various temperatures two things are noticed.

First that the total intensity is a function of the fouth power of the temperature. Secondly, a wavelength, 8max, corresponding to a maximum radiant intensity was observed for each temperature. But as the temperature increases, the wavelength corresponding to the maximum intensity decreases, according to the expression: 8max x T = 2.898 x 10-3 m K

Hence, the intensity of electromagnetic waves depends upon the temperature. The experiment can be simplified by using a blackbody as the source of radiation, which is completely independant on any radiation falling on the body, i.e.the black-body does not reflect any part of the radiaiton that falls on it. From this experiment, four things can be conferred. First that the chamber is full of electromagnetic standing waves with nodes at the walls. Secondly the number of such standing waves with wavelengths between 8 and 8+)8 can be expressed as: (8BV) / 84 Thirdly, each wave contributes a certain fraction given by kT, which is the total energy of an oscillating atom, to the total radiation of the black-body where T is the temperature of its walls. Fourthly, R(8) = (8B/84)kT(c/4), where R(8) is the radiant intensity. This formula is known as the Rayleigh-Jeans formula. But the results predicted by this formula at lower wavelength, are way off the curve observed by experiment. This was known as the UV catastrophe, and made many scientists uncomfortable at that time. The problem was finally solved by Max Planck, by using the assumption that energy is transmitted not as a continous flow, but as discrete packets, known as quanta, each with an energy of h<, where < is the wavelength.

It is not just the waves that show particle-like behaviour. It is true the other way round also. Even particles have been found to some wave-like properties.

Here, we come back to the deBroglie's equation 8 = h/p Since, in our world, macroscopic particles, moving with the smallest of speeds also have a measurable momentum, p, that tends to be large because of the considerable mass, deBroglie wavelengths of large objects are minute and impossible to measure. However, for atomic particles , having small momentum, the deBroglie wavelength can be calculated.

But, this wavelength cannot be seen by simple observation. For a particle to display such a property it must be treated like a wave. Experiments like the double-slit experminent performed by Young actually can only be explained by making this assumption. Such experiments produced clear evidence of the wave phenomena.

Scientists today accept light both as a partcle as well as a wave. As mentioned earlier, the aspect of nature displayed by an object only depends upon the experiment that is performed, and is not independant of it. In other words, "If an experiment is an interrogation of nature, then one gets what he interrogated for".