Physics is where JEE ranks are usually won or lost, since a single conceptual slip changes the whole answer. Questions here span mechanics, rotational motion, thermodynamics, waves and oscillations, electrostatics, current electricity, magnetism, optics, and modern physics. Numerical problems carry full derivations so you can see which step you skipped, not just which option was right.
The rest mass energy of an electron is approximately:
Answer: A
Using E=mc², the rest mass energy of electron (m=9.11×10⁻³¹ kg) is 0.511 MeV, a fundamental constant in modern physics.
Q.2Hard
In photoelectric effect, the stopping potential V₀ for a metal is 2V. If the frequency of incident light is doubled, the new stopping potential will be:
Answer: D
Using hf = Φ + eV₀, when frequency doubles: h(2f) = Φ + eV'₀. Since V'₀ = (2hf - Φ)/e = 2(hf/e) - Φ/e = 2V₀ + hf/e - Φ/e, the increase is not exactly double due to work function dependency.
Q.3Easy
The de Broglie wavelength of a particle is given by:
Answer: A
de Broglie proposed that λ = h/p, where h is Planck's constant and p is momentum. This is fundamental to wave-particle duality.
Q.4Medium
An electron transitions from n=3 to n=1 in a hydrogen atom. The frequency of emitted photon is (Use Rydberg constant R = 1.097×10⁷ m⁻¹):
Iron-56 has the maximum binding energy per nucleon (~8.8 MeV/nucleon), making it the most stable nucleus. This is a key concept in nuclear physics.
Q.6Easy
A radioactive nucleus has half-life of 10 days. After 30 days, the fraction of original nuclei remaining is:
Answer: C
Number of half-lives = 1030 = 3. Remaining fraction = (21)³ = 81
Q.7Medium
The threshold frequency for a metal is f₀. If light of frequency 2f₀ is incident, the maximum kinetic energy of photoelectrons is:
Answer: A
At threshold: hf₀ = Φ. For frequency 2f₀: KEₘₐₓ = h(2f₀) - Φ = 2hf₀ - hf₀ = hf₀
Q.8Easy
Compton scattering demonstrates:
Answer: B
Compton scattering shows that photons behave as particles with momentum, providing evidence for particle nature of light. Wavelength shift Δλ = (h/mₑc)(1 - cosθ).
Q.9Easy
The energy of a photon in terms of wavelength is:
Answer: A
Since c = fλ, we have f = c/λ. Therefore E = hf = hc/λ
Q.10Medium
A charged particle is accelerated through a potential difference of 100V. Its de Broglie wavelength is λ₁. If accelerated through 400V, the wavelength becomes λ₂. The ratio λ₁/λ₂ is:
Answer: B
λ = h/√(2mKE). Since KE ∝ V, λ ∝ 1/√V. Therefore λ₁/λ₂ = √(100400) = √4 = 2
Q.11Medium
The activity of a radioactive sample decreases by 50% in 1 hour. Its half-life is:
Answer: B
Activity A = λN. When activity decreases by 50% in 1 hour, this means N (and A) reduced to half in 1 hour, which is exactly the definition of half-life.
Q.12Medium
The work function of a metal is 2.3 eV. The metal will exhibit photoelectric effect with light of wavelength:
