Which scientist discarded postulate of ether? 

Electric field of plane electromagnetic wave propagating through a non-magnetic medium is given by ${E}=20 \cos \left(2 \times 10^{10} {t}-200 {x}\right) \,{V} / {m} .$ The dielectric constant of the medium is equal to :

(Take $\mu_{{r}}=1$ )

The electric field of a plane electromagnetic wave propagating along the $x$ direction in vacuum is $\overrightarrow{ E }= E _{0} \hat{ j } \cos (\omega t – kx )$. The magnetic field $\overrightarrow{ B },$ at the moment $t =0$ is :

The electric fields of two plane electromagnetic plane waves in vacuum are given by

$\overrightarrow{\mathrm{E}}_{1}=\mathrm{E}_{0} \hat{\mathrm{j}} \cos (\omega \mathrm{t}-\mathrm{kx})$ and

$\overrightarrow{\mathrm{E}}_{2}=\mathrm{E}_{0} \hat{\mathrm{k}} \cos (\omega \mathrm{t}-\mathrm{ky})$

At $t=0,$ a particle of charge $q$ is at origin with a velocity $\overrightarrow{\mathrm{v}}=0.8 \mathrm{c} \hat{\mathrm{j}}$ ($c$ is the speed of light in vacuum). The instantaneous force experienced by the particle is 

The electric field in an electromagnetic wave is given by $E =56.5 \sin \omega( t – x / c ) \;NC ^{-1}$. Find the intensity of the wave if it is propagating along $x-$axis in the free space. (Given $\left.\varepsilon_{0}=8.85 \times 10^{-12} \;C ^{2} N ^{-1} m ^{-2}\right)$