An electromagnetic wave of frequency n = 3.0 MHz passes from vacuum into a dielectric medium with permittivity ε = 4.0. Then

wave length is halved and frequency remains unchanged

Velocity of light, v = $\frac{1}{\sqrt{\mu_0 \varepsilon_0 \varepsilon_{r}}}=\frac{1}{\sqrt{\mu_0 \varepsilon_0 . 4}}=\frac{c}{2}$

v remains constant hence

$\lambda=\frac{v}{v}=\frac{\lambda_0}{2}$ since $v=\frac{c}{2}$

∴ wavelength is halved and frequency remains unchanged.