When wavefronts pass from denser medium to rarer medium, the width of the wavefront

The relation between total magnetic field (B), magnetic intensity $(\mathrm{H})$, permeability of free space $\left(\mu_0\right)$ and susceptibility $(\chi)$ is

In common emitter transistor amplifier, load resistance is $6.5 \mathrm{k} \Omega$ and an input resistance is $1.3 \mathrm{k} \Omega$. If the current gain is 78 , the voltage gain is

A particle having a charge 50 e is revolving in a circular path of radius 0.4 m with $1 \mathrm{r} . \mathrm{p} . \mathrm{s}$. The magnetic field produced at the centre of the circle is $\left(\mu_0=4 \pi \times 10^{-7}\right.$ SI units and $e=1.6 \times 10^{-19} \mathrm{c}$)