Question

An electromagnetic wave of frequency $\upsilon = 3MHz$passes from vacuum into a dielectric medium with permittivity $\varepsilon$ =4 ,Then :

• A. Wavelength and frequency both become half.
• B. Wavelength is doubled and frequency remains unchanged.
• C. Wavelength and frequency both remain unchanged.
• D. Wavelength is halved and frequency remains unchanged.

Answer 1

Answer: (D)

Wavelength is halved and frequency remains unchanged.

Answer 2

: The frequency of electromagnetic wave remains unchanged but the wavelength of electromagnetic wave changes when it passes from one medium to another.

$c = \frac{1}{\sqrt{\mu_{0}\varepsilon_{0}}}$

$\therefore c \propto \frac{1}{\sqrt{\varepsilon_{0}}}$ and $v \propto \frac{1}{\sqrt{\varepsilon}}$

$\therefore\frac{c}{v} = \sqrt{\frac{\varepsilon}{\varepsilon_{0}}} = \sqrt{\frac{4}{1}} = 2$

$\frac{c}{v} = \frac{\upsilon\lambda}{\upsilon\lambda'} = \frac{\lambda}{\lambda'} = 2$ or $\lambda' = \frac{\lambda}{2}$