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Modern Physics

1. Nature of light

It was a matter of great interest for scientists of know that what exactly from the light is made up of or how the light behaves. This is briefly described over here

1.1 Newton's Corpuscular theory :

Newton was the first scientist who said that light is made 0 up tiny elastic particles called "Corpuscles" which travels with the velocity of light. So according to Newtons, light is a particle.

1.2 Huygen's wave theory :

Huygen was a scientist working parallel to Newton who come with a drastically different idea for nature of light & said that light is not a particle but a wave.

1.3 Maxwell's electromagnetic wave theory :

During the time of Huygen, his views regarding nature of light were not accepted as newton was a popular scientist of his time. but, when maxwell asserted that light is a electromagnetic wave, scientists started believing that light is a wave.

1.4 Max Planck's quantum theory of light :

Once again when scientists started believing that the light is a wave max plank came with different idea & asserted that light is not a wave but a photon (i.e. a particle) which he proved through black body radiation spectrum. At this time there was a great confusion about the nature of light which was solved by de-broglie from where origin of theory of matter wave come into picture.

1.5 Debroglie Hypothesis

It supports dual nature of light (wave nature and particle nature). According to him the light consists of particles associated with definite amount of energy and momentum. These particles were later named as photons.

The photon posses momentum and is given by

Modern Physics | Physics Class 12 - NEET ...(1)

P = momentum of one photon

λ = wavelength of wave.

h = Plank's constant = 6.62 × 10-34 Js.

A photon is a packet of energy. It posses energy given by

Modern Physics | Physics Class 12 - NEET ...(2)

where c = speed of light

Debroglie relates particle property (momentum) with wave property (wavelength) i.e. he favours dual nature of light.

Electron volt It is the energy gained by an electron when it is accelerated through a potential difference of one volt.

1 eV = 1.6 × 10-19 Joule.

Now from eq. (2)

Modern Physics | Physics Class 12 - NEET in Joule.

Modern Physics | Physics Class 12 - NEET

Modern Physics | Physics Class 12 - NEET

where λ is in Å

Properties of Photon :

1. Photon travels with speed of light.

2. The rest mass of a photon is zero.

3. There is no concept of photon conservation.

4. All the photons of a particular frequency or wavelength posses the same energy irrespective of the intensity of the radiation.

5. The increase in the intensity of the radiation imply an increase in the number of photon's crossing a given area per second.

  •  When light travels from one medium to another medium then

frequency = const (because it is the property of source)

but v, λ changes

Ex.1 A beam of light having wavelength l and intensity 1 falls normally on an area A of a clean surface then find out the number of photon incident on the surface. 

Modern Physics | Physics Class 12 - NEET

Sol. Total energy incident in time t = I A t

Energy of one photon  E =Modern Physics | Physics Class 12 - NEET

Then number of photon incident in time t  Modern Physics | Physics Class 12 - NEET

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FAQs on Modern Physics - Physics Class 12 - NEET

1. What is modern physics?
Ans. Modern physics is a branch of physics that deals with the study of the behavior of matter and energy at the atomic and subatomic levels. It encompasses theories and concepts such as quantum mechanics, relativity, particle physics, and cosmology.
2. How does modern physics differ from classical physics?
Ans. Modern physics differs from classical physics in several ways. Classical physics describes the behavior of matter and energy on a macroscopic scale, whereas modern physics focuses on the microscopic scale. Additionally, classical physics follows deterministic principles, while modern physics introduces probabilistic concepts. Lastly, classical physics does not account for relativistic effects or quantum phenomena, which are central to modern physics.
3. What are the key principles of quantum mechanics?
Ans. Quantum mechanics is a fundamental theory in modern physics that describes the behavior of particles at the atomic and subatomic levels. Its key principles include wave-particle duality, where particles can exhibit both wave-like and particle-like properties; superposition, which states that particles can exist in multiple states simultaneously; and quantum entanglement, where the properties of two or more particles become interconnected.
4. How does relativity theory impact modern physics?
Ans. Relativity theory, developed by Albert Einstein, is a fundamental theory in modern physics that revolutionized our understanding of space, time, and gravity. It consists of two main theories: special relativity and general relativity. Special relativity deals with the behavior of objects moving at high speeds, while general relativity describes the gravitational interactions between massive objects. The theory of relativity has profound implications for cosmology, black holes, and the nature of the universe.
5. What are the current frontiers of modern physics research?
Ans. Modern physics is a dynamic field with ongoing research in various areas. Some of the current frontiers include the study of dark matter and dark energy, which make up the majority of the universe but remain largely mysterious; the search for a unified theory that combines quantum mechanics and general relativity; and the exploration of quantum computing and quantum information processing. Additionally, advancements in particle physics, astrophysics, and nanotechnology continue to push the boundaries of our understanding of the physical world.
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