Problem — True or false on EM waves (1)

Problem

Determine whether each statement is true or false. (a) An electromagnetic wave in vacuum has $\abs{\vv E}/\abs{\vv B} = c$. (b) Red light has a longer wavelength than blue light. (c) Electromagnetic waves are longitudinal. (d) An electromagnetic wave carries both energy and momentum.

Solution

(a) True. In vacuum the magnitudes of the fields of an EM wave are related at every instant by $\abs{\vv E} = c\,\abs{\vv B}$, so their ratio is exactly the speed of light: $\abs{\vv E}/\abs{\vv B} = c$.

(b) True. In the visible spectrum the wavelength increases from violet/blue ($\sim 450\;\text{nm}$) towards red ($\sim 700\;\text{nm}$). Red has a longer $\lambda$ (and lower frequency) than blue.

(c) False. EM waves are transverse: the fields $\vv E$ and $\vv B$ oscillate perpendicular to the direction of propagation (and to each other).

(d) True. An EM wave carries energy (described by the Poynting vector) and also momentum: the latter is responsible for radiation pressure.

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

Collegamenti

Argomenti: Onde elettromagnetiche Concetti: Onda elettromagnetica