Question 7: EM waves

Question

Column - I lists the waves of the electromagnetic spectrum. Column - II gives approximate frequency range of these waves. Match Column - I and Column - II and choose the correct match from the given choices.

Column I

Column II

(A)
Radiowaves
(P)
$$10^{18} 	ext { to } 10^{20} \mathrm{~Hz}$$

(B)
Microwaves
(P)
$$10^{11} 	ext { to } 5 	imes 10^{14} \mathrm{~Hz}$$

(C)
Infrared
(R)
$$10^4 	ext { to } 10^8 \mathrm{~Hz}$$

(D)
X-rays
(S)
$$10^9 	ext { to } 10^{12} \mathrm{~Hz}$$

Accepted Answer

When an electromagnetic wave passes from one medium to another, the frequency remains unchanged. Here's why:
In vacuum, the speed of light is $$c$$ and the wavelength is given by the formula:
$$\lambda_0 = \frac{c}{f}$$
When the wave enters the dielectric medium, its speed becomes:
$$v = \frac{c}{\sqrt{\epsilon_r \mu_r}}$$
Given that $$\mu_r = 1$$ and $$\epsilon_r = 4.0$$, we have:
$$v = \frac{c}{\sqrt{4.0}} = \frac{c}{2}$$
The frequency $$f$$ remains the same as in vacuum, so the new wavelength $$\lambda'$$ in the medium is:
$$\lambda' = \frac{v}{f} = \frac{c/2}{f} = \frac{1}{2}\left(\frac{c}{f}\right) = \frac{\lambda_0}{2}$$
Thus, the wavelength is halved, and the frequency remains unchanged.
The correct option is:
Option D
Wavelength is halved and frequency remains unchanged.

Answer

(A)-(R)
(B)-(P)
(C)-(S)
(D)-(Q)

Answer

(A)-(R)
(B)-(S)
(C)-(Q)
(D)-(P)

Answer

(A)-(R)
(B)-(S)
(C)-(P)
(D)-(Q)

Answer

(A)-(R)
(B)-(Q)
(C)-(S)
(D)-(P)

EM waves

Choose the correct answer: