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An electromagnetic wave of frequency 0.3 MHz passes from vacuum into a dielectric medium with permittivity 0.4. Then

1. Wavelength is doubled and the frequency remains unchanged
2. Wavelength is doubled and frequency becomes half
3. Wavelength is halved and frequency remains unchanged
4. Wavelength and frequency both remain unchanged

Correct answer: Wavelength is halved and frequency remains unchanged

Solution

When an electromagnetic wave enters a dielectric, its frequency remains unchanged because it is determined by the source. The speed becomes $v = c/\sqrt{\varepsilon_r}$, so for $\varepsilon_r = 4$, the speed becomes half. Since $\lambda = v/f$, the wavelength is also halved.

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