NEET UG Physics Practice Paper – Part 6 (Modern Physics)

NEET UG Physics Practice Paper – Part 6 (Modern Physics)

Submit

NEET UG Physics Notes – Modern Physics (Part 6)

Modern Physics is one of the most scoring and conceptually rich sections for NEET UG. The questions in this quiz primarily revolve around photoelectric effect, atomic models, nuclear physics, and radiation phenomena. Understanding the core principles behind these topics is essential for solving both theoretical and numerical questions efficiently.

---

1. Photoelectric Effect

The photoelectric effect explains the emission of electrons from a metal surface when light of sufficient frequency falls on it. This phenomenon strongly supports the particle nature of light.

Key Concepts:

• Threshold Frequency (ν₀): Minimum frequency required to eject electrons.
• Work Function (φ): Minimum energy needed to remove an electron from a metal surface.
• Einstein’s Photoelectric Equation: $$hf = KE_{max} + \phi$$hf=KEmax​+ϕ

Important Observations:

• Increasing frequency increases kinetic energy.
• Increasing intensity increases the number of emitted electrons.
• Photoelectric emission is instantaneous, proving energy is quantized.

---

2. Photon Theory of Light

According to Planck’s quantum theory:

• Energy is emitted in discrete packets called photons.
• Energy of a photon: $$E = hf$$E=hf
• Momentum of photon: $$p = \frac{h}{\lambda} = \frac{E}{c}$$p=λh​=cE​

This explains phenomena like:

• Photoelectric effect
• Compton scattering

---

3. Dual Nature of Matter

Louis de Broglie proposed that matter also behaves like waves.

De Broglie Wavelength:

$$\lambda = \frac{h}{p} = \frac{h}{mv}$$λ=ph​=mvh​

Experimental Proof:

• Davisson-Germer experiment confirmed electron diffraction.

Implication:

• All moving particles have wave nature, but it is noticeable only at microscopic levels.

---

4. Bohr’s Atomic Model

Bohr proposed a model for hydrogen atom explaining discrete energy levels.

Key Postulates:

• Electrons revolve in fixed orbits (energy levels).
• Energy levels are quantized: $$E_n = -\frac{13.6}{n^2} \, eV$$En​=−n213.6​eV

Bohr Radius:

$$r_n \propto n^2$$rn​∝n2

Spectral Lines:

• Emission occurs when electron jumps from higher to lower energy level.
• Absorption occurs for reverse transitions.

---

5. Atomic Spectra

When electrons transition between energy levels, photons are emitted or absorbed.

Types of Spectra:

• Emission Spectrum: Bright lines on dark background.
• Absorption Spectrum: Dark lines on bright background.

Reason:

Energy difference between levels:$$\Delta E = hf$$ΔE=hf

---

6. Nuclear Structure

The nucleus consists of protons and neutrons (nucleons).

Nuclear Radius:

$$R = R_0 A^{1/3}$$R=R0​A1/3

Where:

• $A$A = mass number
• $R_0 \approx 1.3 \times 10^{-15} \, m$R0​≈1.3×10−15m

---

7. Mass Defect and Binding Energy

Mass Defect:

Difference between actual mass of nucleus and sum of individual nucleon masses.

Binding Energy:

Energy required to break nucleus into individual nucleons.$$E = \Delta m \, c^2$$E=Δmc2

Importance:

• Determines stability of nucleus
• Higher binding energy → more stable nucleus

---

8. Radioactive Decay

Radioactive substances decay spontaneously.

Decay Law:

$$N = N_0 e^{-\lambda t}$$N=N0​e−λt

Where:

• $N$N = number of atoms at time $t$t
• $\lambda$λ = decay constant

Half-Life:

$$T_{1/2} = \frac{0.693}{\lambda}$$T1/2​=λ0.693​

Activity:

$$A = \lambda N$$A=λN

---

9. Types of Radioactive Decay

Alpha Decay (α):

• Emits helium nucleus (2p + 2n)
• Low penetration, high ionization

Beta Decay (β):

• Emits electron (β⁻) or positron (β⁺)
• Moderate penetration

Gamma Decay (γ):

• Emits electromagnetic radiation
• High penetration, no charge

---

10. Nuclear Reactions

Nuclear Fission:

• Splitting of heavy nucleus into smaller nuclei
• Releases large energy and neutrons
• Used in nuclear reactors

Nuclear Fusion:

• Combining light nuclei into heavier nucleus
• Requires extremely high temperature
• Occurs in stars like the Sun

---

11. Energy Production in the Sun

The Sun produces energy through nuclear fusion, primarily:$$4H \rightarrow He + energy$$4H→He+energy

Key Points:

• Enormous energy released due to mass defect
• Supports life on Earth

---

12. Rutherford’s Atomic Model

Rutherford’s gold foil experiment concluded:

• Atom has a small dense nucleus
• Most of the atom is empty space
• Electrons revolve around nucleus

---

13. Fundamental Particles

Electron:

• Charge: $-1.6 \times 10^{-19} \, C$−1.6×10−19C
• Very small mass

Proton:

• Positive charge
• Located in nucleus

Neutron:

• Neutral particle
• Adds mass but no charge

---

14. Mass-Energy Equivalence

Einstein’s famous relation:$$E = mc^2$$E=mc2

Implication:

• Mass can be converted into energy
• Basis of nuclear energy

---

15. Important Exam Insights

Frequently Tested Concepts:

• Photoelectric equation
• De Broglie wavelength
• Bohr energy levels
• Radioactive decay formulas
• Binding energy

Common Mistakes:

• Confusing intensity with frequency
• Misinterpreting half-life formulas
• Ignoring units in Planck’s constant

---

Conclusion

Modern Physics is a high-yield, formula-based, and concept-driven topic for NEET UG. Most questions are direct applications of formulas or conceptual understanding rather than lengthy calculations. By mastering:

• Photoelectric effect
• Atomic models
• Nuclear physics
• Radioactivity laws

you can easily secure full marks in this section.

Focus on understanding why phenomena occur, not just memorizing formulas. This will help you tackle tricky conceptual questions with confidence.