Question: Q. 1. (i) How are electromagnetic waves produced ? Explain.
(ii) A plane electromagnetic wave is travelling through a medium along the +ve $z$-direction. Depict the electromagnetic wave showing the directions of the oscillating electric and magnetic fields.
U] [Foreign I, II, III 2017]
Show Answer
Solution:
Ans. (i) Try yourself, Similar to Q. 2, Short Answer Type Questions-I
(ii) Try yourself, Similar to Q. 9, Short Answer Type Questions-I.
[AI Q. 2. How are EM waves produced by oscillating charges ? Draw a sketch of linearly polarized EM waves propagating in the Z-direction. Indicate the directions of the oscillating electric and magnetic fields.
OR
Write Maxwell’s generalization of Ampere’s Circuital Law. Show that in the process of charging a capacitor, the current produced within the plates of the capacitor is $i=\varepsilon_{0} \frac{d \phi_{\varepsilon}}{d t}$
Where $\phi_{\varepsilon}$ is the electric flux produced during charging of the capacitor plates.
U[Delhi I, II, III 2016]
Ans. (i) Try yourself, Similar to Q. 9, Short Answer Type Questions-I
OR Ampere’s circuital law is given by as $\oint \vec{B} \cdot \overrightarrow{d l}=\mu_{0} I_{c}$ But for a circuit containing capacitor, during its charging / discharging the current within the plates of the capacitor varies, (producing displacement current $i_{d}$ ).
Therefore, the above equation, as generalized by Maxwell, is given as $\oint \vec{B} \cdot \overrightarrow{d l}=\mu_{0} I_{C}+\mu_{0} I_{D} \quad 1$
During the process of charging of capacitor, electric flux $\left(\phi_{\varepsilon}\right)$ between the plates of capacitor changes with time, which produces the current within the plates of capacitor. This current, being proportional to $\frac{d \phi_{\varepsilon}}{d t}$, we have
$$ \begin{equation*} i=\varepsilon_{0} \frac{d \phi_{\varepsilon}}{d t} \tag{1} \end{equation*} $$
[CBSE Marking Scheme 2016]
TOPIC-2
Electromagnetic Spectrum
Revision Notes
Electromagnetic spectrum
Classification of EM-waves is based on their frequency or wavelength range.
- EM radiations are classified as per the frequency and wavelength of wave such as radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, $\mathrm{X}$-rays and gamma rays.
Types of Electromagnetic waves (radio waves, microwaves, infrared, visible, ultraviolet, $X$-rays, gamma rays)
Dlectromagnetic waves require no medium to travel or propagate.
Varying Electric and magnetic fields create electromagnetic waves.
Electromagnetic waves are transverse waves which measures their amplitude, wavelength, or distance between highest/lowest points.
- In electromagnetic waves a crest is the highest point of the wave and through the lowest point of wave in a cycle.
$a=$ Amplitude
$b=$ wavelength
Electromagnetic spectrum is divided into following regions
The electromagnetic spectrum is the distribution of electromagnetic radiation in terms of energy, frequency, or wavelength. The electromagnetic radiation can be described as a stream of photons travelling in a wave like pattern, at the speed of light.
| Type of Radiation | Frequency Range | Wavelength Range |
|---|---|---|
| Gamma rays | $<3 \times 10^{20}$ | $<1 \mathrm{fm}$ |
| $X$-rays | $3 \times 10^{17}-3 \times 10^{20}$ | $1 \mathrm{fm}-1 \mathrm{~nm}$ |
| Ultraviolet | $7.5 \times 10^{14}-3 \times 10^{17}$ | $1 \mathrm{~nm}-400 \mathrm{~nm}$ |
| Visible | $4 \times 10^{14}-5 \times 10^{14}$ | $0.4 \mu \mathrm{m}-0.75 \mu \mathrm{m}$ |
| Near-infrared | $10^{14}-7.5 \times 10^{14}$ | $0.75 \mu \mathrm{m}-3.0 \mu \mathrm{m}$ |
| Midwave infrared | $5 \times 10^{13}-10^{14}$ | $3.0 \mu \mathrm{mm}-6 \mu \mathrm{mm}$ |
| Long wave infrared | $2 \times 10^{3}-5 \times 10^{13}$ | $6.0 \mu \mathrm{m}-15 \mu \mathrm{m}$ |
| Extreme infrared | $3 \times 10^{13}-2 \times 10^{13}$ | $15 \mu \mathrm{mm}-15 \mu \mathrm{mm}$ |
| Micro and radio waves | $<3 \times 10^{11}$ | $>1 \mathrm{~mm}$ |
Uses of Electromagnetic waves
| Band Designation | Applications |
|---|---|
| Audible | Acoustics |
| Extremely Low Frequency (ELF) Radio | Electronics, Submarine Communications |
| Infra Low Frequency (ILF) | Not applicable |
| Very Low Frequency (VLF) Radio | Navigation, Weather |
| Low Frequency (LF) Radio | Navigation, Maritime Communications, Information and Weather Systems, Time Systems |
| Medium Frequency (MF) Radio | Navigation, AM Radio, Mobile Radio |
| High Frequency (HF) Radio | Citizens Band Radio, Mobile Radio, Maritime Radio |
| Very High Frequency (VHF) Radio | Amateur (Ham) Radio, VHF TV, FM Radio, Mobile Satellite, Mobile Radio, Fixed Radio |
| Ultra High Frequency (UHF) Radio | Microwave, Satellite, UHF TV, Paging, Cordless Telephone, Cellular and PCS Telephony, Wireless LAN (WiFi) |
| Super High Frequency (SHF) Radio | Microwave, Satellite, Wireless LAN (WiFi) |
| Extremely High Frequency (EHF) Radio | Microwave, Satellite, Radiolocation |
| Infrared Light (IR) | Wireless LAN Bridges, Wireless LANs, Fiber Optics Remote control |
| Visible Light | Photographic plate, photocells. |
|---|---|
| Ultraviolet (UV) | Photocells, kill bacteria and germs. |
| $X$-Rays | In medicell, Geiger tubes, ionisation chamber. |
| Gamma and Cosmic Rays | In medical (cancer cell killing) |
Types of Electromagnetic waves, wavelength range, Production and Detection :
| Type of Radiation | Wavelength range | Production | Detection |
|---|---|---|---|
| Radio | $>1.0 \times 10^{-1} \mathrm{~m}$ | Rapid acceleration and decelerations of electrons in aerials |
Receiver’s aerials |
| Microwave | $0.1 \mathrm{~m}-1.0 \times 10^{-3} \mathrm{~m}$ | Klystron valve or magnetron valve |
Point contact diodes |
| Infra-red | $1.0 \times 10^{-3} \mathrm{~m}-700 \times 10^{-9} \mathrm{~m}$ | Vibration of atoms and molecules | Thermopiles Bolometer, Infrared photographic film |
| Light | $700 \times 10^{-9} \mathrm{~m}-400 \times 10^{-9} \mathrm{~m}$ | Electrons in atoms emit light when they move formoneenergy level to a lower energy level |
The eyes Photocells Photographic film |
| Ultraviolet | $400 \times 10^{-9} \mathrm{~m}-1.0 \times 10^{-9} \mathrm{~m}$ | Inner shell electrons in atoms moving from one energy level to a lower level |
Photocells Photographic film |
| $X$-rays | $1.0 \times 10^{-9} \mathrm{~m}-1.0 \times 10^{-12} \mathrm{~m}$ | $X$-ray tubesor inner shell electrons | Photographic film Geiger tubes Ionisation chamber |
| Gamma rays | $<1.0 \times 10^{-12} \mathrm{~m}$ | Radioactive decay of the nucleus | Photographic film Geiger tubes Ionisation chamber |
Know the Terms
- Electromagnetic waves : Waves that appear from changing of electric and magnetic fields.
- Gamma rays : Rays with smallest wayelengths and highest frequencies having high energy capable of travelling long distances through air andaremost penetrating.
$X$-rays : These are the rays with long and small wavelengths having higher energy as compared to ultraviolet radiation.
Ultraviolet (UV) radiation : It is a part of electromagnetic spectrum that lies between X-rays and visible light.
V Visible light : It is avisible spectrum which is part of electromagnetic spectrum which can be seen by hum eyes.
- Infrared (IR) radiation: These are thermal radiation which is part of electromagnetic spectrum that lies between visible light and microwaves.
Radio waves : Waves with long wavelengths used in television, cell phone and radio communications.
