Physics
Mechanics Thermodynamics Optics Electrostatics & Current Magnetism & EMI Modern Physics
Module 1 · JEE Physics

Modern Physics

Photoelectric effect, atomic models, nuclear physics, semiconductors, logic gates, communication.
Photoelectric Effect · Atomic Models · Nuclear Physics · Semiconductors
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Learning Objectives

  • Understand the fundamental concepts of Modern Physics
  • Apply key formulas and techniques to solve problems
  • Practice with exam-level questions to build speed and accuracy

Key Concepts

What is Modern Physics?

Modern physics covers the revolutionary developments in physics from the early 20th century, including quantum mechanics, relativity, atomic and nuclear physics. For NEET, key topics include photoelectric effect, Bohr's atomic model, radioactivity, nuclear fission and fusion, and dual nature of matter. These concepts explain phenomena at atomic and subatomic scales.

Key Concepts

Photoelectric Effect: Einstein's equation: hf = φ + KEmax, where φ is work function. Threshold frequency f0 = φ/h. Bohr's Model: Angular momentum mvr = nh/2π, Energy En = -13.6/n2 eV, Radius rn = 0.529 n2/Z Å. de Broglie Wavelength: λ = h/p = h/(mv). Radioactive Decay: N = N0e-λt, half-life T1/2 = 0.693/λ. Mass-Energy Equivalence: E = mc2. Nuclear Binding Energy: Δm × c2.

Solved Examples

Example 1
The work function of a metal is 2.3 eV. Light of wavelength 400 nm falls on it. Find the maximum kinetic energy of emitted photoelectrons. (h = 6.63 × 10-34 Js, c = 3 × 108 m/s, 1 eV = 1.6 × 10-19 J)

a) 0.8 eV b) 1.1 eV c) 1.5 eV d) 2.3 eV
Solution: Energy of photon E = hc/λ = (6.63 × 10-34 × 3 × 108)/(400 × 10-9) = 4.97 × 10-19 J = 4.97 × 10-19/1.6 × 10-19 = 3.1 eV. KEmax = E - φ = 3.1 - 2.3 = 0.8 eV. Hence option (a) is correct.
Example 2
In a hydrogen atom, find the energy of the electron in the second excited state (n = 3).

a) -3.4 eV b) -1.51 eV c) -0.85 eV d) -13.6 eV
Solution: En = -13.6/n2 eV. For n = 3, E3 = -13.6/9 = -1.51 eV. Hence option (b) is correct.
Example 3
A radioactive sample has a half-life of 5 days. How much fraction of the sample remains after 15 days?

a) 1/2 b) 1/4 c) 1/8 d) 1/16
Solution: Number of half-lives n = t/T1/2 = 15/5 = 3. Fraction remaining = (1/2)n = (1/2)3 = 1/8. Hence option (c) is correct.

Shortcut Techniques

  • For photoelectric effect, remember that intensity affects number of photoelectrons (current), not their kinetic energy.
  • For atomic spectra, the number of spectral lines when an electron drops from n to 1 is n(n-1)/2.
  • In radioactive decay, after n half-lives, remaining fraction = (1/2)n and decayed fraction = 1 - (1/2)n.
  • For binding energy calculations, mass defect Δm is converted to energy using 1 amu = 931 MeV.
Pro Tip
For NEET, memorize the key constants: h = 6.63 × 10-34 Js, 1 eV = 1.6 × 10-19 J, 1 amu = 931 MeV/c2. Also remember that the ground state energy of hydrogen is -13.6 eV.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip
Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip
Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip
Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip
Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Practice Questions

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