MODERN PHYSICS BASICS – COMPLETE
NOTES
1. Introduction to Modern Physics
Modern physics refers to the scientific concepts that emerged after 1900, reshaping our
understanding of reality.
It includes:
● Quantum Physics → behavior of electrons, atoms, photons
● Relativity → nature of space, time, energy
● Atomic & Nuclear Physics → structure and behavior of atoms and nuclei
● Particle Physics → fundamental particles like quarks and leptons
Classical physics failed to explain key phenomena like:
● Why metals glow when heated
● Why electrons don’t fall into the nucleus
● Why the speed of light is constant
● Why radioactive decay happens
Modern physics answers these questions.
2. Blackbody Radiation
A blackbody is a perfect absorber and emitter of radiation.
Classical Failure
, Classical theory predicted “ultraviolet catastrophe” — infinite energy at high frequencies.
Planck’s Quantum Solution
Energy is emitted in discrete packets called quanta:
E=hfE = hfE=hf
Where
● EEE = energy of a photon
● hhh = Planck’s constant
● fff = frequency
This was the beginning of quantum theory.
3. The Photoelectric Effect
Einstein discovered that:
● Light behaves as particles called photons.
● Electrons are ejected from metals when hit by light of sufficient frequency, not
intensity.
Key Idea
A minimum energy called work function is needed to remove an electron.
Ephoton=hfE_{\text{photon}} = hfEphoton=hf
This confirmed that light has dual nature: both wave and particle.
4. Wave-Particle Duality
Every particle has a wavelength:
NOTES
1. Introduction to Modern Physics
Modern physics refers to the scientific concepts that emerged after 1900, reshaping our
understanding of reality.
It includes:
● Quantum Physics → behavior of electrons, atoms, photons
● Relativity → nature of space, time, energy
● Atomic & Nuclear Physics → structure and behavior of atoms and nuclei
● Particle Physics → fundamental particles like quarks and leptons
Classical physics failed to explain key phenomena like:
● Why metals glow when heated
● Why electrons don’t fall into the nucleus
● Why the speed of light is constant
● Why radioactive decay happens
Modern physics answers these questions.
2. Blackbody Radiation
A blackbody is a perfect absorber and emitter of radiation.
Classical Failure
, Classical theory predicted “ultraviolet catastrophe” — infinite energy at high frequencies.
Planck’s Quantum Solution
Energy is emitted in discrete packets called quanta:
E=hfE = hfE=hf
Where
● EEE = energy of a photon
● hhh = Planck’s constant
● fff = frequency
This was the beginning of quantum theory.
3. The Photoelectric Effect
Einstein discovered that:
● Light behaves as particles called photons.
● Electrons are ejected from metals when hit by light of sufficient frequency, not
intensity.
Key Idea
A minimum energy called work function is needed to remove an electron.
Ephoton=hfE_{\text{photon}} = hfEphoton=hf
This confirmed that light has dual nature: both wave and particle.
4. Wave-Particle Duality
Every particle has a wavelength:
