Question 1

The speed of electromagnetic waves in vacuum is approximately 3 × 10⁸ m/s. If the frequency of an electromagnetic wave is 6 × 10¹⁴ Hz, what is its wavelength?

Accepted Answer

Using c = fλ, wavelength λ = c/f = (3 × 10⁸)/(6 × 10¹⁴) = 5 × 10⁻⁷ m.

Question 2

Which of the following correctly represents the relationship between electric field (E), magnetic field (B), and velocity (c) in an electromagnetic wave?

Accepted Answer

In an electromagnetic wave, the ratio of electric field amplitude to magnetic field amplitude equals the speed of light: E/B = c.

Question 3

An electromagnetic wave has a wavelength of 4 × 10⁻⁷ m in vacuum. To which region of the electromagnetic spectrum does it belong?

Accepted Answer

Wavelength 4 × 10⁻⁷ m (400 nm) corresponds to violet light in the visible spectrum (380–750 nm).

Question 4

The frequency of an electromagnetic wave is 2 × 10¹⁵ Hz. What is the energy of a single photon of this wave? (Use h = 6.6 × 10⁻³⁴ J·s)

Accepted Answer

Photon energy E = hf = (6.6 × 10⁻³⁴) × (2 × 10¹⁵) = 1.32 × 10⁻¹⁸ J.

Question 5

Which statement correctly describes a property of electromagnetic waves?

Accepted Answer

Electromagnetic waves consist of perpendicular E and B fields oscillating in phase with each other and perpendicular to the direction of propagation.

Question 6

The relationship between the speed of electromagnetic waves (c), electric field (E), and magnetic field (B) in vacuum is given by:

Accepted Answer

In an electromagnetic wave, the ratio of electric field amplitude to magnetic field amplitude equals the speed of light: c = E/B or E = cB.

Question 7

An electromagnetic wave has a frequency of 5 × 10¹⁴ Hz. What is its wavelength in vacuum? (Use c = 3 × 10⁸ m/s)

Accepted Answer

Using λ = c/f, we get λ = (3 × 10⁸)/(5 × 10¹⁴) = 6 × 10⁻⁷ m, which lies in the visible light region.

Question 8

Which of the following correctly represents the relationship between electric field (E), magnetic field (B), and the direction of propagation in an electromagnetic wave?

Accepted Answer

In an electromagnetic wave, the electric field, magnetic field, and direction of propagation are mutually perpendicular to each other.

Question 9

The energy density of an electromagnetic wave in vacuum is given by u = ε₀E². If the electric field amplitude is doubled, by what factor does the energy density increase?

Accepted Answer

Since energy density u ∝ E², when E is doubled, u becomes 4 times larger (u ∝ (2E)² = 4E²).

Question 10

An electromagnetic wave propagates in a medium with refractive index n = 1.5. If the frequency is 6 × 10¹⁴ Hz, find its wavelength in the medium. (c = 3 × 10⁸ m/s)

Accepted Answer

Speed in medium v = c/n = (3 × 10⁸)/1.5 = 2 × 10⁸ m/s; wavelength λ = v/f = (2 × 10⁸)/(6 × 10¹⁴) = 3.33 × 10⁻⁷ m ≈ 4 × 10⁻⁷ m.

Question 11

Which electromagnetic wave has the shortest wavelength?

Accepted Answer

Gamma rays have the shortest wavelength (< 10⁻¹² m) among all electromagnetic radiations in the spectrum.

Question 12

The intensity of an electromagnetic wave is proportional to the square of its amplitude. If the amplitude of an EM wave is reduced to one-third of its original value, the intensity becomes:

Accepted Answer

Since I ∝ A², when amplitude is reduced to A/3, intensity becomes I/9 (I ∝ (A/3)² = A²/9).

Question 13

The speed of electromagnetic waves in a vacuum is independent of which of the following?

Accepted Answer

The speed of EM waves in vacuum is always c = 3 × 10⁸ m/s, regardless of frequency or wavelength; it only depends on the permittivity and permeability of the medium.

Question 14

An electromagnetic wave of wavelength 4 × 10⁻⁷ m travels through a glass slab (n = 1.6). What is the frequency of the wave in the glass? (c = 3 × 10⁸ m/s)

Accepted Answer

Frequency remains constant in any medium; f = c/λ₀ = (3 × 10⁸)/(4 × 10⁻⁷) = 7.5 × 10¹⁴ Hz (where λ₀ is the vacuum wavelength).

Question 15

An electromagnetic wave travels in a medium with refractive index 1.5. If the frequency of the wave is 6 × 10¹⁴ Hz, what is the wavelength of the wave in the medium? (Take c = 3 × 10⁸ m/s)

Accepted Answer

Wavelength in medium λ = c/(n×f) = (3×10⁸)/(1.5×6×10¹⁴) = 2.22×10⁻⁷ m.

Question 16

Which of the following correctly describes the relationship between electric field E, magnetic field B, and velocity v of propagation in an electromagnetic wave in vacuum?

Accepted Answer

In an EM wave, E and B are mutually perpendicular, both perpendicular to the direction of propagation, and their amplitude ratio is E/B = c.

Question 17

An electromagnetic wave has a wavelength of 500 nm in vacuum. When this wave enters a medium with refractive index 1.33, what is the frequency of the wave in the medium? (Take c = 3 × 10⁸ m/s)

Accepted Answer

Frequency is independent of medium; f = c/λ = (3×10⁸)/(500×10⁻⁹) = 6.0×10¹⁴ Hz remains constant.

Question 18

The electric field amplitude of an electromagnetic wave is 120 V/m. What is the corresponding magnetic field amplitude? (Take c = 3 × 10⁸ m/s)

Accepted Answer

In an EM wave, B₀ = E₀/c = 120/(3×10⁸) = 4.0×10⁻⁷ T.

CDS Electromagnetic Waves

Electromagnetic Waves — Practice Questions