Which of the following correctly represents the relationship between wave velocity, frequency, and wavelength?

The correct answer is B: v = fλ. The fundamental wave equation is v = fλ, where v is velocity, f is frequency, and λ is wavelength

At thermal equilibrium in a semiconductor, the product of electron and hole concentrations is:

The correct answer is B: ni² (intrinsic carrier concentration squared). The mass action law states ne·nh = ni² at thermal equilibrium, regardless of doping type. This is a fundamental relationship derived from Fermi-Dirac statistics.

The ratio of specific charge (e/m) for an α-particle to that of a proton is:

The correct answer is A: 1/2. α-particle: charge = 2e, mass = 4u. Proton: charge = e, mass = u. (e/m)ₐ/(e/m)ₚ = (2e/4u)/(e/u) = 1/2

The electric field inside a uniformly charged spherical shell of radius R and charge Q at a distance r from center (r < R) is:

The correct answer is B: E = 0. By Gauss's law, electric field inside a uniformly charged spherical shell is zero everywhere

The intensity of a sound wave is I. If the amplitude is doubled and frequency is halved, the new intensity will be:

The correct answer is B: 2I. Intensity ∝ A²f². New intensity = (2A)² × (f/2)² × I/(A²f²) = 4 × (1/4) × I = 2I

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Modern Physics

Physics questions for JEE Main — Mechanics, Electrostatics, Optics, Modern Physics.

48 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 41–48 of 48

Topics in JEE Physics

All Mechanics 100 Thermodynamics 100 Electrostatics 100 Current Electricity 100 Magnetism 100 Optics 100 Modern Physics 100 Semiconductors 100 Waves 100

Q.41 Medium Modern Physics

According to Heisenberg's uncertainty principle, if the uncertainty in position (Δx) is zero, then uncertainty in momentum (Δp) will be:

A Zero

B Finite and measurable

C Infinite

D Undefined

Correct Answer: C. Infinite

EXPLANATION

ΔxΔp ≥ h/4π. If Δx = 0, then Δp must be ≥ ∞ to satisfy the inequality, making momentum completely uncertain.

Test

Q.42 Medium Modern Physics

The characteristic X-ray spectrum is produced due to:

A Bremsstrahlung radiation

B Transition of electrons from higher to lower shells

C Compton scattering

D Pair production

Correct Answer: B. Transition of electrons from higher to lower shells

EXPLANATION

Characteristic X-rays arise from inner-shell electron transitions (e.g., L→K shell). Each element has specific wavelengths, hence 'characteristic'.

Test

Q.43 Medium Modern Physics

The energy of an α-particle in the ground state of hydrogen-like atom (Z=2) compared to ground state of hydrogen is:

A Same

B 4 times

C 8 times

D 16 times

Correct Answer: B. 4 times

EXPLANATION

For hydrogen-like atoms: E = -13.6Z²/n² eV. For He⁺ (Z=2): E = -13.6×4 = -54.4 eV. For H (Z=1): E = -13.6 eV. Ratio is 4:1

Test

Q.44 Medium Modern Physics

The work function of a metal is 2.3 eV. The metal will exhibit photoelectric effect with light of wavelength:

A 700 nm

B 600 nm

C 400 nm

D 300 nm

Correct Answer: D. 300 nm

EXPLANATION

Threshold wavelength λ₀ = hc/Φ = (1240 eVnm)/(2.3 eV) ≈ 539 nm. Photoelectric effect occurs for λ < 539 nm. Only 300 nm satisfies this.

Test

Q.45 Medium Modern Physics

The activity of a radioactive sample decreases by 50% in 1 hour. Its half-life is:

A 30 minutes

B 1 hour

C 2 hours

D Cannot be determined

Correct Answer: B. 1 hour

EXPLANATION

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.

Test

Q.46 Medium Modern Physics

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:

A 1/2

B 2

C 1/4

D 4

Correct Answer: B. 2

EXPLANATION

λ = h/√(2mKE). Since KE ∝ V, λ ∝ 1/√V. Therefore λ₁/λ₂ = √(400/100) = √4 = 2

Test

Q.47 Medium Modern Physics

The threshold frequency for a metal is f₀. If light of frequency 2f₀ is incident, the maximum kinetic energy of photoelectrons is:

A hf₀

B 2hf₀

C 3hf₀

D Cannot be determined without knowing work function

Correct Answer: A. hf₀

EXPLANATION

At threshold: hf₀ = Φ. For frequency 2f₀: KEₘₐₓ = h(2f₀) - Φ = 2hf₀ - hf₀ = hf₀

Test

Q.48 Medium Modern Physics

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):

A 2.93×10¹⁵ Hz

B 2.47×10¹⁵ Hz

C 1.89×10¹⁵ Hz

D 3.27×10¹⁵ Hz

Correct Answer: A. 2.93×10¹⁵ Hz

EXPLANATION

Using f = cR(1/n₁² - 1/n₂²) = 3×10⁸ × 1.097×10⁷ × (1/1 - 1/9) = 3×10⁸ × 1.097×10⁷ × (8/9) ≈ 2.93×10¹⁵ Hz

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