NEET Physics: Electromagnetic Waves questions with solutions

Q1. Who named the negatively charged particles present in cathode rays as electrons?

1. J.J. Thomson
2. Stoney
3. Goldstein
4. None of these

Answer: J.J. Thomson

J.J. Thomson studied cathode rays and concluded they were made of tiny negatively charged particles. He is credited with identifying and naming these particles as electrons.

Q2. Name the high energetic invisible electromagnetic wave which helps in the study of structure of crystals.

1. \( X \) -rays
2. Cathod rays
3. Gamma rays
4. All of the above

Answer: \( X \) -rays

X-rays have wavelengths comparable to interatomic distances, so they can diffract from crystal planes and produce patterns used to determine crystal structure. Cathode rays are streams of electrons, and gamma rays are too penetrating and not typically used for this purpose.

Q3. Which of the following type of radiations are radiated by on oscillating electric charge?

1. Electric
2. Magnetic
3. Thermoelectric
4. Electromagnetic

Answer: Electromagnetic

An oscillating electric charge continuously changes its electric field, which induces a changing magnetic field. These linked fields propagate away as electromagnetic radiation.

Q4. Microwaves are detected by

1. barometer
2. point contact diodes
3. thermopiles
4. the eye

Answer: point contact diodes

Point contact diodes are sensitive enough to rectify microwave-frequency signals, making them suitable detectors. The other options measure pressure, heat, or visible light, not microwaves.

Q5. Threshold wavelength of tungsten is 2300 angstrom. If ultraviolet light of wavelength 1800 angstrom is incident on it, then the maximum kinetic energy of photoelectrons would be?

1. 1.5 ev
2. 2.5ev
3. 3.0 ev
4. \( 5.0 \mathrm{eV} \)

Answer: 2.5ev

The threshold wavelength gives the work function of tungsten, and the incident ultraviolet light has higher photon energy because its wavelength is shorter. The maximum kinetic energy is the difference between incident photon energy and work function, which comes out to 2.5 eV.

Q6. Name the rays or waves of wavelength nearly \( 0.1 n m \)

1. IR rays
2. UV rays
3. Gamma rays
4. x-rays

Answer: Gamma rays

A wavelength of about 0.1 nm is extremely short, placing it in the gamma-ray region of the electromagnetic spectrum. Gamma rays have the shortest wavelengths and highest frequencies among the listed options.

Q7. According to Maxwell's hypothesis, changing of electric filed give rise to

1. magnetic field
2. pressure gradient
3. charge
4. voltage

Answer: magnetic field

Maxwell proposed that a changing electric field produces a magnetic field, which completes the symmetry of electromagnetism. This is the idea behind electromagnetic waves, where changing electric and magnetic fields sustain each other.

Q8. An electromagnetic wave has energy in the form of

1. variable electric field
2. variable magnetic field
3. both A and B
4. none of the above

Answer: both A and B

An electromagnetic wave carries energy through oscillating electric and magnetic fields. These two fields are perpendicular to each other and to the direction of propagation, so the energy is in both fields, not just one.

Q9. When light propagates in vaccum there is an electric field and a magnetic field. Which of the following is not true about these field?

1. They are constant in time
2. They have zero average value
3. They are perpendicular to the direction of propagation of light
4. They are mutually perpendicular

Answer: They are constant in time

In vacuum, light is an electromagnetic wave with electric and magnetic fields that vary sinusoidally in time and space. So they are not constant in time, while they do have zero average value and are perpendicular to each other and to the direction of propagation.

Q10. When light propagates in vacuum there is an electric field and a magnetic field. These fields This question has multiple correct options

1. are constant in time
2. have zero average value
3. are perpendicular to the direction of propagation of light
4. are mutually perpendicular

Answer: are mutually perpendicular

In a vacuum, light is an electromagnetic wave with electric and magnetic fields oscillating perpendicular to each other. They are not constant in time, and the statement about zero average value is not the defining property here.

Q11. In photoelectric effect, the photocurrent

1. depends both on intensity and frequency of the incident light.
2. does not depend on the frequency of incident light but depends on the intensity of the incident light.
3. decreases with increase in frequency of incident light.
4. increases with increase in frequency of incident light.

Answer: does not depend on the frequency of incident light but depends on the intensity of the incident light.

In the photoelectric effect, photocurrent is proportional to the number of photoelectrons emitted per second, which depends on the light intensity. Once the frequency is above threshold, changing frequency mainly changes the kinetic energy of electrons, not the current.

Q12. The electricity produced by creating photo electrons is called:

1. Photoelectricity
2. Static electricity
3. Thermal electricity
4. Piezoelectricity

Answer: Photoelectricity

When light strikes a material and ejects electrons, the resulting electrical effect is called photoelectricity. The other options involve charge buildup, heat, or pressure, not light-induced electron emission.

Q13. The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is : (c = speed of electromagnetic waves)

1. 1:1
2. 1:c
3. c:1
4. 1:c²

Answer: 1:1

In an electromagnetic wave, the electric field and magnetic field contribute equally to the intensity, as intensity is proportional to the square of the amplitudes of both fields.

Q14. Out of the following options which one can be used to produce a propagating electromagnetic wave?

1. A charge moving at constant velocity
2. A stationary charge
3. A chargeless particle
4. An accelerating charge

Answer: An accelerating charge

An accelerating charge produces a time-varying electric field, which in turn generates a time-varying magnetic field, leading to the propagation of an electromagnetic wave.

Q15. In an electromagnetic wave in free space the root mean square value of the electric field is \( E_{rms} = 6 \mathrm{V/m} \). The peak value of the magnetic field is

1. 2.83 × 10⁻⁸ T
2. 0.70 × 10⁻⁸ T
3. 4.23 × 10⁻⁸ T
4. 1.41 × 10⁻⁸ T

Answer: 2.83 × 10⁻⁸ T

The peak value of the magnetic field is related to the peak value of the electric field by the equation: \( B_0 = \frac{E_0}{c} \), where \( E_0 = \sqrt{2} E_{rms} \) and \( c \) is the speed of light. Substituting \( E_{rms} = 6 \mathrm{V/m} \), \( E_0 = 6\sqrt{2} \), and \( c = 3 \times 10^8 \mathrm{m/s} \), we get \( B_0 = \frac{6\sqrt{2}}{3 \times 10^8} = 2.83 \times 10^{-8} \mathrm{T} \).

Q16. Light with an energy flux of \( 25 \times 10^4 \mathrm{Wm}^{-2} \) falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm², the average force exerted on the surface is:

1. 1.25 × 10⁻⁶ N
2. 2.50 × 10⁻⁶ N
3. 1.20 × 10⁻⁶ N
4. 3.0 × 10⁻⁶ N

Answer: 2.50 × 10⁻⁶ N

The force exerted by light on a perfectly reflecting surface is given by the formula: F = 2(P/c), where P is the power incident on the surface and c is the speed of light. First, calculate the power: P = energy flux × area = (25 × 10⁴) × (15 × 10⁻⁴) = 375 W. Then, F = 2 × (375 / 3 × 10⁸) = 2.50 × 10⁻⁶ N.

Q17. An electromagnetic wave of frequency \( \nu = 3.0 \mathrm{MHz} \) passes from vacuum into a dielectric medium with relative permittivity \( \varepsilon_r = 4.0 \). Then

1. wavelength is doubled and frequency is unchanged
2. wavelength is doubled and frequency becomes half
3. wavelength is halved and frequency remains unchanged
4. wavelength and frequency both remain unchanged

Answer: wavelength is halved and frequency remains unchanged

When an electromagnetic wave enters a dielectric medium, its frequency remains unchanged, but its wavelength decreases by a factor of the square root of the relative permittivity. Here, the wavelength is halved because the square root of 4 is 2.

Q18. The ratio of amplitude of magnetic field to the amplitude of electric field for an electromagnetic wave propagating in vacuum is equal to:

1. the speed of light in vacuum
2. reciprocal of speed of light in vacuum
3. the ratio of magnetic permeability to the electric susceptibility of vacuum
4. unity

Answer: reciprocal of speed of light in vacuum

For an electromagnetic wave in vacuum, the ratio of the amplitude of the magnetic field to the amplitude of the electric field is equal to the reciprocal of the speed of light (1/c). This follows from the relationship between electric and magnetic fields in electromagnetic waves.

Q19. The electric and the magnetic field associated with an E.M. wave, propagating along the +z-axis, can be represented by:

1. [E̅ = E₀î, B̅ = B₀ĵ]
2. [E̅ = E₀k̂, B̅ = B₀î]
3. [E̅ = E₀ĵ, B̅ = B₀k̂]
4. [E̅ = E₀k̂, B̅ = B₀k̂]

Answer: [E̅ = E₀î, B̅ = B₀ĵ]

In an electromagnetic wave propagating along the +z-axis, the electric field (E̅) and magnetic field (B̅) are perpendicular to each other and to the direction of propagation. Here, E̅ = E₀î and B̅ = B₀ĵ satisfy this condition.

Q20. The electric field associated with an e.m. wave in vacuum is given by E̅ = î 40 cos (kz − 6 × 10⁸t), where E̅, z and t are in volt/m, meter and seconds respectively. The value of wave vector k is:

1. 2 m⁻¹
2. 0.5 m⁻¹
3. 6 m⁻¹
4. 3 m⁻¹

Answer: 3 m⁻¹

The wave equation is of the form E = E₀ cos(kz - ωt). Here, ω = 6 × 10⁸ rad/s. The wave vector k is related to ω by the relation k = ω/c, where c = 3 × 10⁸ m/s. Substituting, k = (6 × 10⁸) / (3 × 10⁸) = 2 m⁻¹.