An electromagnetic wave of frequency 2 MHz propagates from vacuum...

In vacuum wavelength, \lambda_0=$\frac{c}{f_0}=\frac{3 \times 10^8}{2 \times 10^6}=$1.5 $\times$10^2=150 $\mathrm{~m}$In medium, speed of wave is v=$\frac{\mathcal{c}}{\sqrt{\varepsilon_r \mu_r}} \approx \frac{\mathcal{c}}{\sqrt{\varepsilon_r}} ($as medium is non-magnetic) \therefore \quad v=$\frac{\mathcal{c}}{\sqrt{9}}=\frac{\mathcal{c}}{3}=$1 $\times$10^8 $\mathrm{~ms}^{-1}$Frequency remaining same, wavelength in medium is $\lambda=\frac{v}{f_0}=\frac{10^8}{2 \times 10^6}=50 \mathrm{~m}\therefore$ Wavelength decreases by $100 \mathrm{~m}$.