Question

The electromagnetic waves travel with

• A. the same speed in all media
• B. the speed of sound in free space
• C. the speed of light \( c = 3 \times 10^{8} \) ms⁻¹ in solid medium
• D. the speed of light \( c = 3 \times 10^{8} \) ms⁻¹ in fluid medium
• E. the speed of light \( c = 3 \times 10^{8} \) ms⁻¹ in free space

Hint:

Remember that while the speed and wavelength of an EM wave change when it enters a different medium, its \textbf{frequency} always remains constant.

Answer 1

Answer: (E)

Step 1: Understanding the Concept:
Electromagnetic (EM) waves consist of oscillating electric and magnetic fields. A defining characteristic of EM waves is that they do not require a material medium to propagate; they can travel through a vacuum (free space).
Step 2: Key Formula or Approach:
The speed of electromagnetic waves in a vacuum is a fundamental constant, given by the formula: \[ c = \frac{1}{\sqrt{\mu_0 \epsilon_0}} \approx 3 \times 10^8 \text{ m/s} \] where \( \mu_0 \) is the permeability of free space and \( \epsilon_0 \) is the permittivity of free space. In any material medium, the speed \( v \) is given by \( v = \frac{c}{n} \), where \( n \) is the refractive index of the medium (\( n>1 \)).
Step 3: Detailed Explanation:
Let's analyze the given options:
- (A) is incorrect because the speed depends on the medium's refractive index. EM waves slow down when entering a denser medium.
- (B) is incorrect because sound is a mechanical wave that cannot travel in free space, meaning its speed is zero there, whereas EM waves travel at their maximum speed.
- (C) and (D) are incorrect because light slows down when it enters a solid or fluid medium due to their refractive indices being greater than 1.
- (E) is correct. Electromagnetic waves, including light, travel at their maximum speed \( c = 3 \times 10^8 \text{ m/s} \) in free space (a vacuum).
Step 4: Final Answer:
The electromagnetic waves travel with the speed of light \( c = 3 \times 10^8 \text{ ms}^1 \) in free space.