What are called non-mechanical waves? Give its examples.

In a plane electromagnetic wave,which of the following has/have a zero average value in one complete cycle?
$(a)$ Magnetic field
$(b)$ Magnetic energy
$(c)$ Electric field
$(d)$ Electric energy

$A$ plane electromagnetic wave is incident on a material surface. If the wave delivers momentum $p$ and energy $E$,then

The electric field of a plane electromagnetic wave, travelling in an unknown non-magnetic medium is given by, $E_y = 20 \sin(3 \times 10^6 x - 4.5 \times 10^{14} t) \text{ V/m}$ (where $x, t$ and other values are in $S$.$I$. units). The dielectric constant of the medium is . . . . . . . (Speed of light in free space is $c = 3 \times 10^8 \text{ m/s}$)

The electric field of a plane electromagnetic wave of wave number $k$ and angular frequency $\omega$ is given by $\vec{E} = E_0(\hat{i} + \hat{j}) \sin(kz - \omega t)$. Which of the following gives the direction of the associated magnetic field $\vec{B}$?

The speed of an electromagnetic wave in a vacuum depends upon the source of radiation.