A plane electromagnetic wave of frequency 500 MHz is travelling in vacuum along $y - $direction. At a particular point in space and time, $\overrightarrow B = 8.0 \times {10^{ - 8}}\widehat zT$. The value of electric field at this point is: <br> (speed of light $ = 3 \times {10^8}m{s^{ - 1}}$). $\widehat x,\widehat {\,y},\widehat {\,z}$ are unit vectors along $x,\,y$ and $z$ directions.