The work function of a metal is 3 eV. What is the threshold frequency for photoelectric emission?

The correct answer is B: 7.2×10¹⁴ Hz. Threshold frequency: f₀ = W/h = 3/(6.63×10⁻³⁴) = 4.5×10¹⁴ Hz. Wait, recalculating: f₀ = 3 eV/(4.14×10⁻¹⁵ eVs) ≈ 7.2×10¹⁴ Hz.

An electric dipole of moment p is placed in a uniform electric field E at angle θ to the field. The potential energy is:

The correct answer is A: -pE cos θ. Potential energy of dipole in electric field: U = -p·E = -pE cos θ. Minimum at θ = 0 (aligned).

A hydrogen atom transitions from n=4 to n=2 state. The wavelength of emitted photon is approximately:

The correct answer is A: 486 nm. Using Rydberg formula: 1/λ = R(1/4 - 1/16) = 3R/16. With R = 1.097×10⁷ m⁻¹, we get λ ≈ 486 nm (H-beta line).

An electric dipole of dipole moment p is placed in a uniform electric field E at an angle θ to the field. What is the torque acting on the dipole?

The correct answer is A: τ = pE sin θ. The torque on a dipole in a uniform field is τ = p × E = pE sin θ, which tends to align the dipole with the field.

A stationary observer hears a sound from an approaching source. The observed frequency is:

The correct answer is B: Greater than the actual frequency. Doppler effect: When source approaches observer, f' = f(v)/(v-vs), which is greater than f

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JEE Physics
Waves

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

37 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 31–37 of 37

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.31 Easy Waves

When a longitudinal wave travels through a medium, particles undergo:

A Transverse oscillation only

B Longitudinal oscillation only

C Circular motion

D Random motion

Correct Answer: B. Longitudinal oscillation only

EXPLANATION

In longitudinal waves, particles oscillate parallel to the direction of wave propagation, creating compressions and rarefactions

Test

Q.32 Easy Waves

The velocity of a transverse wave on a string depends on:

A Only frequency

B Only tension and mass per unit length

C Only amplitude

D Frequency and wavelength only

Correct Answer: B. Only tension and mass per unit length

EXPLANATION

Wave velocity v = √(T/μ), where T is tension and μ is mass per unit length. It is independent of frequency and amplitude

Test

Q.33 Easy Waves

Standing waves are formed when:

A Two waves of different frequencies travel in the same direction

B Two waves of the same frequency travel in opposite directions

C A single wave is reflected

D Waves refract through a medium

Correct Answer: B. Two waves of the same frequency travel in opposite directions

EXPLANATION

Standing waves result from the superposition of two coherent waves traveling in opposite directions with the same frequency

Test

Q.34 Easy Waves

A string of length L is fixed at both ends. The fundamental frequency is f₁. What is the frequency of the second harmonic?

A f₁

B 2f₁

C 3f₁

D f₁/2

Correct Answer: B. 2f₁

EXPLANATION

For a string fixed at both ends, fₙ = nf₁ where n is the harmonic number. Second harmonic (n=2) has f₂ = 2f₁

Test

Q.35 Easy Waves

Two waves of the same frequency interfere constructively. The resultant amplitude is:

A The difference of individual amplitudes

B The sum of individual amplitudes

C Zero

D Equal to one of the amplitudes

Correct Answer: B. The sum of individual amplitudes

EXPLANATION

In constructive interference, phase difference is 0° or 2πn. Resultant amplitude = A₁ + A₂

Test

Q.36 Easy Waves

Which of the following is a characteristic of transverse waves?

A Particles vibrate parallel to the direction of wave propagation

B Particles vibrate perpendicular to the direction of wave propagation

C No energy is transferred

D They can only travel through solids

Correct Answer: B. Particles vibrate perpendicular to the direction of wave propagation

EXPLANATION

In transverse waves, particle oscillation is perpendicular to the direction of wave propagation, such as in electromagnetic waves and ripples on water

Test

Q.37 Easy Waves

A wave travels with a velocity of 300 m/s and has a frequency of 150 Hz. What is its wavelength?

A 2 m

B 4 m

C 0.5 m

D 6 m

Correct Answer: A. 2 m

EXPLANATION

Using v = fλ, where v = 300 m/s and f = 150 Hz, λ = v/f = 300/150 = 2 m

Test

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Waves