Electromagnetic Waves MCQ Questions & Answers in Electrostatics and Magnetism | Physics

$$\lambda = \frac{c}{v} = \frac{{3 \times {{10}^8}}}{{8.2 \times {{10}^6}}} = 36.5\,m$$

$$\eqalign{ & \oint {\vec E} \cdot d\vec \ell = - \frac{{d{\phi _B}}}{{dt}} \cr & {\text{or}}\,\,E\left( {2\pi r} \right) = \frac{d}{{dt}}\left( {kt \times \pi {r^2}} \right) \cr & \therefore E = \frac{{kr}}{2} \cr} $$

The wavelength order of waves are given below

Waves	Wavelength (in $$\mathop {\text{A}}\limits^ \circ $$)
(a) X-rays	$$0.001\mathop {\text{A}}\limits^ \circ $$  to $$100\mathop {\text{A}}\limits^ \circ $$
(b) Ultaviolet rays	$$10\,\mathop {\text{A}}\limits^ \circ $$  to $$4000\,\mathop {\text{A}}\limits^ \circ $$
(c) $$\gamma $$-rays	$$0.0001\,\mathop {\text{A}}\limits^ \circ $$  to $$1\,\mathop {\text{A}}\limits^ \circ $$
(d) Cosmic rays	upto $$4 \times {10^{ - 3}}\mathop {\text{A}}\limits^ \circ $$

Thus, cosmic rays have the minimum wavelength.

The green house is a room made of glass which is constructed to grow plants in open space. The glass transmits the infrared radiations of short wavelength and the visible light, but it absorbs the infrared radiations of long wavelength. During the day time, the light and the infrared radiations of short wavelength enter inside the green house through the roof and the walls and they are absorbed by the plants.
At night, plants emit the infrared radiations through the glass walls and roof. As a result, green house remains warm. This is called green house effect.

As we know that,
As the displacement current through the capacitor is,
$${I_d} = {I_c} = \frac{{dq}}{{dt}}\,......\left( {\text{i}} \right)$$
As given that, $$q = {q_0}\cos 2\pi vt$$
Putting this value of $$q$$ in Eq (i), we get
So, $${I_d} = {I_c} = \frac{d}{{dt}}\left( {{q_0}{{\cos }^2}\pi vt} \right)$$
$$\eqalign{ & {I_d} = {I_c} = - {q_0}\sin 2\pi vt \times 2\pi v \cr & {I_d} = {I_c} = 2\pi v{q_0}\sin 2\pi vt \cr} $$

Frequency of radiations given are listed below.

	Waves		Frequency(in Hz)
1.	$$\gamma $$-rays	(a)	$$3 \times {10^{21}}$$  to $$3 \times {10^{18}}$$
2.	X-rays	(b)	$$3 \times {10^{18}}$$  to $$3 \times {10^{16}}$$
3.	Ultraviolet radiation	(c)	$$3 \times {10^{16}}$$  to $$7.5 \times {10^{14}}$$

Thus, correct order is $$a > b > c.$$

Space communication refers to sending, receiving and processing of information through space. Following are the modes of space communication
(i) Ground or surface wave propagation
(ii) Space wave or tropospheric wave propagation
(iii) Sky wave propagation
(iv) Satellite communication

Here, amplitude of electric field, $${E_0} = 100\,V/m;$$    amplitude of magnetic field, $${H_0} = 0.265\,A/m.$$    We know that the maximum rate of energy flow,
$$S = {E_0} \times {H_0} = 100 \times 0.265 = 26.5\,W/{m^2}.$$

Short wave (wavelength $$30\,km$$  to $$30\,cm$$ ). These waves are used for radio transmission and for general communication purpose to a longer distance from ionosphere.
The wavelength of these wave ranges between $$4000\,\mathop {\text{A}}\limits^ \circ $$  to $$100\,\mathop {\text{A}}\limits^ \circ $$  that is smaller wavelength and higher frequency. They are absorbed by atmosphere and convert oxygen into ozone. They cause skin diseases and they are harmful to eye and cause permanent blindness.

(A) $$\beta $$-rays are fast moving electrons. So, they are not electromagnetic waves.
(B) Heat rays can travel through vaccum via radiation process. They are electromagnetic waves.
(C) $$X$$-rays are electromagnetic waves having wavelengths from about $${10^{ - 8}}m$$  to $${10^{ - 12}}m.$$
(D) $$\gamma $$-rays are electromagnetic waves having wavelengths ranging from about $${10^{ - 16}}m$$  to $${10^{ - 14}}m.$$
Hence, choice (A) is correct.