Modern Physics, as the name suggests, involves the study of new concepts. It involves elements of chemistry too. The area of physics extensively studied in school is called classical, or Newtonian, because the principles involved in it were first studied by Isaac Newton. Modern Physics on the other hand, differs from this classical physics, in several ways. Some of these include the following:The theory of relativityIn classical physics, we assume time to be a quantity that is independent of all others. In other words, we assume how fast we are moving, does not affect our perception of time. But, one of the fundamental concepts of relativity is that time can indeed be affected by velocity. The concept states, in technical terms that time and space can be dilated. In simple words, it means that different observers will measure different time periods if they move with a velocity relative to each other. This can also happen if the observers have a gravitational potential difference between them.Wave Particle DualityIn Newtonian physics, we assume light to consist of a stream of particles, which successfully explains the rectilinear motion of light. But at the same time, there are other phenomena associated with light that can not be explained by this. A famous experiment conducted by Thomas Young involving slits proves that light has some wave nature. In modern physics therefore, we assume light to have a dual nature.

To understand this, we have to first confront some crucial theories put forward by the leading scientists of the 20th century.

Planck’s lawPlanck’s law describes electromagnetic radiation. The law is notable for assuming that light (electromagnetic radiation) travels with quantized energy, that is, it only takes certain values.

Photoelectric effectThis effect involves energy of form converting to another. If light falls on certain metals, electrons are released and thus a current flows. Einstein established that this effect depends on only the frequency of the light. A high intensity light may not cause this effect if the frequency is not high enough. Einstein explained that electrons receive energy from the electromagnetic radiation in “discrete portions” and this energy is proportional to the frequency of incident light with the Planck’s constant as the constant of proportionality.

De – Broglie’s contributionsLouis De Broglie said that all matter has a wave nature. This was a revolutionary theory. According to him, wavelength is inversely proportional to momentum, and the constant of proportionality is the Planck’s constant.

The relationship between mass and energyOne of the first things learnt at school, are the principles of conservation of mass, and the conservation of energy. Modern physics however holds mass and energy to be interchangeable. This violates our initial basic belief that neither energy nor mass can be

created nor destroyed. In fact, they can be converted to each other. A very shocking result from this hypothesis is that, in fact, all mass is actually a form of energy, and all energy is a form of mass. The relationship between them was defined by Einstein. We have all heard of the following relation-

E=mC2

Here C is the speed of light in a vacuum; M and E are mass and energy respectively.

A very interesting situation arises from this concept. Suppose we were continuously supply energy to a particle, so much so that its speed approaches the speed of light. However, the object cannot go much faster, even if we keep supplying energy. Therefore we are faced with the absurd situation where the particle’s velocity remains constant, while its energy increases. This proves that the object is actually gaining energy in the form of mass! The value of this mass, depends on the velocity of the observer.

The structure of atom and the basis of quantum mechanicsWe are all aware of the different models of the structure of atom. After Rutherford’s model was comprehensively disproved by Maxwell’s electromagnetic theory, Bohr gave his own famous model of the atom. This can be understood to be the start of quantum mechanics. Now that Bohr’s model has been replaced with more detailed theories, we call his contributions to science, part of the old quantum theory. Nevertheless, it is important for us to rehash the basic principles that caused Bohr’s theory to fail.Heisenberg’s Uncertainty PrincipleThis principle can be understood in a number of ways. It basically states that we cannot accurately determine the position AND momentum of any subatomic particle at a given time. This is due to the fact that the act of our measuring one of the two, changes the value of the other.

Mathematically, it is given by:

Here and represent the deviation in the measurement of position and momentum respectively. h is the Planck’s constant. This principle moves us beyond the classical approach of accurately measuring everything, and instead takes to a world of quantum theory where everything is based on probability.