An electromagnetic wave of frequency ν = 3.0 MHz passes from vaccum into a dielectric medium with relative permittivity εr = 4.0. Then (a) Wavelength is doubled and frequency becomes half. (b) Wavelength is halved and frequency remains unchanged. (c) Wavelength and frequency both remain unchanged. (d) Wavelength is doubled and frequency unchanged.

Answer: (b). Frequency is set by the source, so it does not change when the wave enters a new medium. Refractive index n = √(εr) = √(4) = 2, so speed v = c/2. λ = v/ν ⇒ λ' = λ/2 So the answer is (b) wavelength is halved and frequency remains unchanged.

An electromagnetic wave of frequency ν = 3.0 MHz passes from vaccum… | NEET 2013

Q24NEET 2013Easy

Speed of EM Waves in a Medium

An electromagnetic wave of frequency $\nu = 3.0$ MHz passes from vaccum into a dielectric medium with relative permittivity $\varepsilon_r$ = 4.0. Then

Show answer and solution

Answer: (b)

Frequency is set by the source, so it does not change when the wave enters a new medium.
Refractive index $n = \sqrt{\varepsilon_r} = \sqrt{4} = 2$ , so speed $v = c/2$ .
$\lambda = v/\nu \Rightarrow \lambda' = \lambda/2$
So the answer is (b) wavelength is halved and frequency remains unchanged.

Report a mistake

Speed of EM Waves in a Medium question from NEET 2013

More questions on Speed of EM Waves in a Medium