Instructions
Total Questions: 20 | Marks: 4 each | No Negative Marking
Modern Physics – IIT JEE Notes (Set 20)
Introduction to Modern Physics
Overview
Modern Physics deals with concepts developed after classical physics failed to explain microscopic phenomena. It includes quantum mechanics, atomic physics, nuclear physics, and semiconductor electronics.
Importance
Modern Physics forms the foundation of lasers, semiconductors, nuclear reactors, and electronic devices.
Photoelectric Effect
Definition
The emission of electrons from a metal surface when light of suitable frequency falls on it is called photoelectric effect.
Experimental Observations
Photoelectric emission occurs instantly when frequency exceeds threshold frequency.
Einstein’s Explanation
Einstein explained photoelectric effect using particle nature of light called photons.
Einstein Photoelectric Equation
hν = φ + KEmax
Key Terms
h = Planck’s constant
ν = frequency of incident light
φ = work function
KE = kinetic energy of emitted electrons
Photon
Definition
A photon is a packet of electromagnetic energy.
Energy Formula
E = hν
Momentum Formula
p = h/λ
Threshold Frequency
Definition
Minimum frequency required to eject photoelectrons from a metal surface.
Key Insight
No photoelectric emission occurs below threshold frequency regardless of intensity.
de Broglie Hypothesis
Statement
Every moving particle has wave nature associated with it.
de Broglie Wavelength
λ = h/p
Importance
This established wave-particle duality of matter.
Bohr’s Atomic Model
Main Postulates
Electrons revolve around nucleus only in certain allowed circular orbits without radiating energy.
Angular Momentum Quantization
mvr = nh/2π
Energy Levels
En = -13.6/n² eV
Hydrogen Spectrum
Spectral Series
Lyman, Balmer, Paschen, Brackett, and Pfund series.
Balmer Series
Visible region of hydrogen spectrum.
Rydberg Formula
1/λ = R(1/n₁² – 1/n₂²)
Atomic Radius
Bohr Radius
Radius of first orbit in hydrogen atom is called Bohr radius.
Formula
rn ∝ n²
X-Rays
Production
Produced when high-speed electrons strike a metal target.
Properties
X-rays are electromagnetic waves with very short wavelength and high penetrating power.
Radioactivity
Definition
Spontaneous disintegration of unstable nuclei with emission of radiation.
Discovery
Discovered by Henri Becquerel.
Types of Radioactive Emissions
Alpha Particles
Helium nuclei carrying +2 charge.
Beta Particles
Fast moving electrons.
Gamma Rays
High energy electromagnetic waves.
Radioactive Decay Law
Formula
N = N₀e-λt
Decay Constant
λ represents probability of decay per unit time.
Half-Life
Definition
Time required for half of radioactive nuclei to decay.
Formula
T1/2 = 0.693/λ
Mean Life
Definition
Average lifetime of radioactive nuclei.
Formula
τ = 1/λ
Nuclear Binding Energy
Concept
Energy required to separate nucleus into individual nucleons.
Mass Defect
Difference between actual nuclear mass and sum of masses of nucleons.
Einstein Relation
E = mc²
Nuclear Fission
Definition
Splitting of heavy nucleus into lighter nuclei with release of energy.
Example
Uranium-235 fission.
Applications
Nuclear reactors and atomic bombs.
Nuclear Fusion
Definition
Combination of light nuclei to form heavier nucleus.
Example
Fusion of hydrogen nuclei in the Sun.
Key Insight
Fusion releases more energy per unit mass than fission.
Semiconductors
Definition
Materials having conductivity between conductors and insulators.
Examples
Silicon and Germanium.
Intrinsic and Extrinsic Semiconductors
Intrinsic Semiconductor
Pure semiconductor without impurities.
Extrinsic Semiconductor
Semiconductor doped with impurities to increase conductivity.
p-Type and n-Type Semiconductors
p-Type
Majority charge carriers are holes.
n-Type
Majority charge carriers are electrons.
p-n Junction Diode
Definition
A semiconductor device formed by joining p-type and n-type materials.
Forward Bias
Allows current to flow easily.
Reverse Bias
Opposes current flow.
LED
Full Form
Light Emitting Diode.
Working
Emits light when current passes through it.
Transistor
Function
Used for amplification and switching.
Types
NPN and PNP transistors.
Conceptual Insights
Key Understanding
Modern Physics combines wave and particle nature to explain microscopic phenomena.
Common Mistakes
Students often confuse threshold frequency with intensity and mix up fission and fusion processes.
Important Exam Concepts
Conceptual Traps
Photoelectric current depends on intensity while kinetic energy depends on frequency.
JEE Strategy
Focus on formulas, graphs, and conceptual understanding of photoelectric effect, Bohr model, and semiconductors. Practice numerical problems regularly.