An electromagnetic wave has an electric field given by the expression (in Cartesian coordinates) $\vec E(x,t) = 6.0\,\cos(1 \times 10^7x - 3 \times 10^{15}t)\hat z$. What is the direction of the magnetic field at time $t = 0$ and position $x = 0$?

The efficiency of a bulb of power $60 \ W$ is $16 \%$. The peak value of the electric field produced by the electromagnetic radiation from the bulb at a distance of $2 \ m$ from the bulb is $\left(\frac{1}{4 \pi \epsilon_0}=9 \times 10^9 \ Nm^2 C^{-2}\right)$ (in $Vm^{-1}$)

Answer the following questions:
$(a)$ Long distance radio broadcasts use short-wave bands. Why?
$(b)$ It is necessary to use satellites for long distance $TV$ transmission. Why?
$(c)$ Optical and radio telescopes are built on the ground but $X$-ray astronomy is possible only from satellites orbiting the earth. Why?
$(d)$ The small ozone layer on top of the stratosphere is crucial for human survival. Why?
$(e)$ If the earth did not have an atmosphere,would its average surface temperature be higher or lower than what it is now?
$(f)$ Some scientists have predicted that a global nuclear war on the earth would be followed by a severe 'nuclear winter' with a devastating effect on life on earth. What might be the basis of this prediction?

The velocity factor of a transmission line is $x$. If the dielectric constant of the medium is $2.6$,the value of $x$ is:

If the magnetic field in a plane progressive wave is represented by the equation $B_{y} = 2 \times 10^{-7} \sin(0.5 \times 10^3 x + 1.5 \pi \times 10^{11} t) \ T$,then the frequency of the wave is (In the equation,time $t$ is in seconds).

The velocity of an electromagnetic wave is parallel to: