What is the displacement variable in a simple pendulum and the propagation of light?

The speed of an electromagnetic wave in a medium (whose dielectric constant is $2.25$ and relative permeability is $4$) is equal to .......... $\times 10^8 \, m/s$.

$A$ beam of light travelling along the $X$-axis is described by the electric field $E_{y} = 900 \sin \omega(t - x/c)$. The ratio of the electric force to the magnetic force on a charge $q$ moving along the $Y$-axis with a speed of $3 \times 10^{7} \, m/s$ will be:
[Given: speed of light $c = 3 \times 10^{8} \, m/s$]

$A$ plane electromagnetic $(EM)$ wave is propagating along the $x$-direction. It has a wavelength of $4 \text{ mm}$. If the electric field is in the $y$-direction with a maximum magnitude of $60 \text{ Vm}^{-1}$,the equation for the magnetic field is:

The magnetic field of an electromagnetic wave is given by $\vec B = 1.6 \times 10^{-6} \cos(2 \times 10^7 z + 6 \times 10^{15} t) (2\hat i + \hat j) \text{ Wb/m}^2$. The associated electric field will be

$A$ plane electromagnetic wave is given by $E_z = 100 \cos(6 \times 10^8 t + 4x) \text{ V/m}$. The refractive index of the medium in which the wave is traveling is: