The classical physics is also known as classical mechanics. This was the first branch of physics to be discovered. Classical mechanics explains many concepts in ordinary microscopic scale, whereas Quantum Theory explains the aspects of nature at the subatomic level. Classical physics explains the movement of the parts of machinery, projectile motion and the motion of astronomical objects.
In the early 20th century there were two major discoveries in the field of physics. The first one was Einstein’s theory of relativity and the second was the Quantum Theory, which can also be called Quantum Mechanics. According to the older theories of classical physics, the energy was treated as a continuous phenomenon, while the matter was considered to move in a continuous manner. But quantum theory proposed that energy was in the form of discrete particles called quanta. These packets of energy or quanta are considered to behave like a particle and a wave. Quantum theory and theory of relativity form the basis for modern physics. The principles of quantum mechanics are used in quantum optics, quantum computing, quantum chemistry etc.
Developments in Quantum theory
- In the year 1900, Plank assumed that light energy was made up of individual units
- In 1905, Einstein’s proposed quantum theory. He said not just the energy but the radiation itself was quantised.
- In the year 1924, Louis de Broglie proposed the principle of wave-particle duality. According to the principle depending on the condition the subatomic particles of energy and matter will behave like a particle or wave.
- In 1927, Heisenberg proposed the uncertainty principle. According to this principle, the position and momentum of the subatomic particle cannot be measured simultaneously.
Energy of the Photons
The fact that light energy is made up of small particles is demonstrated by the Photoelectric Effect. The photoelectric effect is the phenomenon seen when electrically charged particles get released from a material after it absorbs electromagnetic radiation. To put it in simple words, it is seen sometimes that light hitting a metallic surface causes electrons to be emitted by it (photoelectrons). When the intensity of such light is increased, it is noticed that more electrons are emitted but there is no increase in the kinetic energy of the electrons. However, when the frequency of the light is increased, the number of electrons (photoelectrons) emitted stays the same but the velocity of the electrons (in other words the kinetic energy of the electrons) increases. When the light is of higher frequency, the electrons that are emitted are more energetic.
The result seen in the photoelectric effect can be explained by considering the light to be consisting of particle-like packets of energy (photons). When the intensity of light is increased, the number of packets (all having the same energy) hitting the metal surface increases. The energy from these photons is used by electrons to escape. However, when the frequency of the light when increased, the number of photons remains the same but the energy carried by each of those packets (photons) increases.