Problem — True or false on EM waves (2)

Problem

True or false?

• (a) Electromagnetic waves propagate in vacuum.
• (b) The fields $\vv E$ and $\vv B$ in a plane wave are perpendicular to each other and to the direction of propagation.
• (c) Visible light is the only electromagnetic wave that carries energy.
• (d) All electromagnetic waves in vacuum propagate at the same speed.
• (e) An electromagnetic wave needs a medium to propagate.

Solution

(a) True. The electromagnetic wave is a varying field $\vv E$ that generates a varying field $\vv B$ and vice versa: it sustains itself without needing matter. In fact, vacuum is the ideal propagation “medium”.

(b) True. The plane electromagnetic wave is transverse: $\vv E \perp \vv B$ and both are perpendicular to the direction of propagation, identified by the product $\vv E \times \vv B$ (direction of the Poynting vector).

(c) False. Every electromagnetic wave carries energy (and momentum), not just visible light: microwaves heat food, X-rays expose plates, radio waves drive aerials.

(d) True. In vacuum all electromagnetic waves, of any frequency, travel at the same speed $c = 3{,}00\cdot 10^8\;\text{m/s} = 1/\sqrt{\mu_0\varepsilon_0}$.

(e) False. Unlike mechanical waves (sound, waves on a string), the electromagnetic wave does not need any material medium: it propagates even in vacuum.

$\ev{\text{(a) T,\ (b) T,\ (c) F,\ (d) T,\ (e) F}}$

Links

Topics: Onde elettromagnetiche Concepts: Onda elettromagnetica