Electric field of a plane electromagnetic wave propagating through a non-magnetic medium is given by $E=20\cos \left(2\times {10}^{10}t-200x\right) \mathrm{V} {\mathrm{m}}^{-1}.$ The dielectric constant of the medium is equal to:
(Take ${\mu }_{\mathbf{r}}=1 )$

A plane electromagnetic wave of frequency $500 \mathrm{MHz}$ is traveling in a vacuum along the $y-$direction. At a particular point in space and time, $\overrightarrow{B}=8.0\times {10}^{-8}\hat{\mathrm{z}} \mathrm{T}$. The value of the electric field at this point is:(speed of light $=3\times {10}^8{ \mathrm{ms}}^{-1}$) $\hat{\mathrm{x}},\hat{\mathrm{y}},\hat{\mathrm{z}}$ are unit vectors along $x,y$ and $z$ direction.

If $2.5\times {10}^{-6} \mathrm{N}$ average force is exerted by a light wave on a non-reflecting surface of $30{ \mathrm{cm}}^2$ area during $40 \min$ of time span, the energy flux of light just before it falls on the surface is _______ $\mathrm{W} {\mathrm{cm}}^{-2}.$(Round off to the Nearest Integer), (Assume complete absorption and normal incidence conditions are there)