Electromagnetic Spectrum

Electromagnetic Waves · CBSE

Theory

What the spectrum is

At the time Maxwell made his prediction, only visible light was familiar as an em wave; ultraviolet and infrared were barely established, and X-rays and gamma rays were discovered by the end of the nineteenth century.
The classification of em waves according to frequency (equivalently, wavelength) is the electromagnetic spectrum. It stretches, in principle, over an infinite range, and in practice from about $10^{-12}$ m (gamma rays) to about $10^{6}$ m (long radio waves).
There is no sharp division between one kind of wave and the next; the classification is based roughly on how the waves are produced and how they are detected. X-rays and gamma rays overlap in wavelength and are distinguished by origin — nuclear for gamma rays, extranuclear (inner-shell electrons or a metal target) for X-rays.
All parts of the spectrum travel through vacuum with the same speed $c$ . The only basic difference between them is wavelength or frequency, and hence their mode of interaction with matter.
They interact with matter through their electric and magnetic fields, which set the charges present in matter oscillating; the details of absorption and scattering depend on the wavelength and on the atoms and molecules of the medium.
The wavelength of the radiation is often correlated with the size of the system that radiates it: gamma rays from nuclei, X-rays from heavy atoms, radio waves from accelerating electrons in circuits. An antenna radiates most efficiently at a wavelength about equal to its own size.

Order of the bands

Increasing wavelength: gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, radio waves.
Increasing frequency (and increasing photon energy $E = h\nu$ ): radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays.
So, for example, microwaves $<$ infrared $<$ ultraviolet $<$ gamma rays in order of increasing frequency, and ultraviolet $<$ infrared $<$ microwaves in order of increasing wavelength.

Master table — the one CBSE examines directly

Radiation	Wavelength range	Frequency range	Production	Detection	Uses
Radio waves	Greater than $0.1$ m, up to about $10^{6}$ m	Less than about $3\times10^{9}$ Hz; broadcast band $500$ kHz to $1000$ MHz	Rapid acceleration and deceleration of electrons in aerials; oscillating LC circuits	Receiver's aerials	Radio and TV communication, AM band $530$ – $1710$ kHz, short wave up to $54$ MHz, TV $54$ – $890$ MHz, FM $88$ – $108$ MHz, cellular phones in the UHF band
Microwaves	$0.1$ m to $1$ mm	About $3\times10^{9}$ Hz to $3\times10^{11}$ Hz; often quoted as $3\times10^{8}$ to $3\times10^{11}$ Hz	Special vacuum tubes — klystron valve, magnetron valve, Gunn diode	Point contact diodes	Radar for aircraft navigation, speed guns for vehicles and fast balls, microwave ovens, long-distance communication
Infrared	$1$ mm to $700$ nm	About $3\times10^{11}$ Hz to $4\times10^{14}$ Hz	Vibration of atoms and molecules; hot bodies; semiconductor LEDs	Thermopiles, bolometer, infrared photographic film	Physical therapy and treatment of muscular strain, remote switches of TV and hi-fi, night vision, photography through haze and fog, greenhouse warming of the Earth, satellite observation of crop growth
Visible light	$700$ nm to $400$ nm	About $4\times10^{14}$ Hz to $7\times10^{14}$ Hz	Electrons in atoms emitting when they move from a higher to a lower energy level	The eye, photocells, photographic film	Vision; it is the part of the spectrum detected by the human eye and gives us information about the world
Ultraviolet	$400$ nm to $1$ nm (NCERT text: $400$ nm down to $0.6$ nm)	About $8\times10^{14}$ Hz to $3\times10^{17}$ Hz; commonly quoted $8\times10^{14}$ to $8\times10^{16}$ Hz	Inner-shell electrons in atoms moving to a lower level; special UV lamps, very hot bodies, the Sun, welding arcs	Photocells, photographic film	Killing germs in water purifiers, LASIK eye surgery, sterilising and preserving foodstuff
X-rays	$1$ nm to $10^{-3}$ nm (NCERT text: $10^{-8}$ m to $10^{-13}$ m)	About $3\times10^{17}$ Hz to $3\times10^{20}$ Hz; commonly quoted $10^{16}$ to $10^{20}$ Hz	X-ray tubes: bombarding a metal target of high atomic number with high-energy electrons; inner-shell electron transitions	Photographic film, Geiger tubes, ionisation chamber	Diagnostic tool in medicine such as detecting bone fractures, treatment of certain cancers, study of crystal structure by X-ray diffraction
Gamma rays	Less than $10^{-3}$ nm; about $10^{-10}$ m down to less than $10^{-14}$ m	Greater than about $3\times10^{18}$ Hz	Radioactive decay of the nucleus; nuclear reactions; decay of certain subatomic particles	Photographic film, Geiger tubes, ionisation chamber	Destroying cancer cells (radiotherapy), sterilisation of medical equipment, nuclear studies

Band-by-band notes

Radio waves

Produced by the accelerated motion of charges in conducting wires (aerials).
Used in radio and television communication systems; generally $500$ kHz to about $1000$ MHz.
AM band $530$ kHz to $1710$ kHz; short-wave bands up to $54$ MHz; TV $54$ MHz to $890$ MHz; FM $88$ MHz to $108$ MHz; cellular phones use the ultrahigh-frequency (UHF) band.
Longest wavelength in the spectrum.

Microwaves

Short-wavelength radio waves with frequencies in the gigahertz range, produced by special vacuum tubes: klystrons, magnetrons and Gunn diodes.
Why suitable for radar: their short wavelength lets them be sent out as a narrow, well-directed beam with little diffraction spreading; the beam reflects off a target and returns, and the short wavelength gives good resolution for the size and position of the object. They also travel through cloud and haze.
Microwave ovens: the frequency is chosen to match the resonant frequency of water molecules, so energy is transferred efficiently into the kinetic energy of the molecules, raising the temperature of any food containing water.
The $21$ cm line emitted by interstellar hydrogen lies in the microwave region.

Infrared

Produced by hot bodies and by the vibration of atoms and molecules; lies just beyond the red end of the visible spectrum.
Called heat waves because water molecules (and CO $_2$ , NH $_3$ and many others) present in most materials readily absorb infrared; the absorbed energy increases the thermal motion of the molecules, raising the internal energy and hence the temperature of the substance and its surroundings.
Neighbours in the spectrum: visible light on the shorter-wavelength side, microwaves on the longer-wavelength side.
Infrared lamps are used in physical therapy and to treat muscular strain.
Greenhouse effect: incoming visible sunlight passes fairly easily through the atmosphere and is absorbed by the Earth's surface, which re-radiates it as longer-wavelength infrared. Greenhouse gases such as carbon dioxide and water vapour trap this infrared, keeping the surface warm. Without an atmosphere the Earth's average surface temperature would be lower, because that infrared would escape into space.
Infrared detectors on Earth satellites are used for military purposes and to observe the growth of crops.
Semiconductor LEDs emit infrared and are used in remote switches of TV sets, video recorders and hi-fi systems.
Infrared is scattered much less than visible light by fog, mist and haze, so it is used for photography and for improving visibility on runways in fog, and in night-vision devices.

Visible light

The most familiar form; the part of the spectrum detected by the human eye, about $4\times10^{14}$ Hz to $7\times10^{14}$ Hz, i.e. $700$ nm to $400$ nm.
Emitted when electrons in atoms move from one energy level to a lower one.
Light emitted or reflected from objects gives us information about the world. Different animals are sensitive to different ranges: snakes detect infrared, and the visible range of many insects extends into the ultraviolet.
The centre of sensitivity of the human eye coincides with the peak of the Sun's wavelength distribution.

Ultraviolet

Covers about $400$ nm down to about $0.6$ nm. Produced by special lamps and by very hot bodies; the Sun is an important source, and welding arcs emit it intensely.
Most solar UV is absorbed by the ozone layer at an altitude of about $40$ – $50$ km. The ozone layer is crucial for survival because it stops harmful UV from reaching the surface; without it UV would penetrate living tissue and cause skin cancer, eye damage and immune suppression. Its depletion by chlorofluorocarbons (CFCs, such as freon) is a matter of international concern.
In large quantities UV harms humans: it induces extra melanin production, causing tanning, and in excess causes sunburn, premature ageing of the skin and skin cancer.
UV is absorbed by ordinary glass, so one cannot get tanned or sunburnt through a glass window. Welders wear special glass goggles or face masks mainly to shield their eyes from the intense UV of the arc.
Because of its short wavelength UV can be focussed into very narrow beams, which is used in high-precision applications such as LASIK (laser-assisted in situ keratomileusis) eye surgery.
UV lamps kill germs in water purifiers, and UV is used to sterilise and preserve foodstuff.

X-rays

Lie beyond the ultraviolet region, from about $10^{-8}$ m ( $10$ nm) down to about $10^{-13}$ m ( $10^{-4}$ nm).
Commonly produced by bombarding a metal target of high atomic number with high-energy electrons, which are suddenly decelerated on striking the target.
Used as a diagnostic tool in medicine — for example to detect fractures in bones — and as a treatment for certain forms of cancer; also to study the structure of crystals by diffraction.
X-rays damage or destroy living tissue, so overexposure must be avoided.

Gamma rays

Highest frequency and shortest wavelength in the spectrum, from about $10^{-10}$ m down to less than $10^{-14}$ m.
Produced in nuclear reactions and emitted by radioactive nuclei and in the decay of certain subatomic particles — they are of nuclear origin.
Used in medicine to destroy cancer cells (radiotherapy), and for sterilisation.

Quick identification table for board one-liners

Clue in the question	Answer	Extra detail usually wanted
Radar, aircraft navigation, speed guns	Microwaves	$3\times10^{8}$ to $3\times10^{11}$ Hz, or $1$ mm to $0.1$ m; short wavelength gives a narrow, well-directed beam
Killing germs in water purifiers	Ultraviolet	$10$ nm to $400$ nm
Eye surgery (LASIK)	Ultraviolet	$8\times10^{14}$ to $8\times10^{16}$ Hz
Welder's protective goggles	Ultraviolet	Emitted intensely by the welding arc
Heat waves; treating muscular strain	Infrared	$700$ nm to $1$ mm; produced by hot bodies
TV remote switches; night vision; seeing through fog	Infrared	Scattered far less than visible light
Earth satellites observing crop growth	Infrared	About $7.5\times10^{-7}$ m to $10^{-3}$ m
Maintaining the Earth's warmth	Infrared	Greenhouse effect
Diagnostic tool in medicine; bone fractures; crystal structure	X-rays	$10^{16}$ to $10^{20}$ Hz; metal target hit by high-energy electrons
Produced by bombarding a metal target with high-speed electrons	X-rays	Bremsstrahlung from sudden deceleration
Destroying cancer cells; highest frequency; nuclear origin	Gamma rays	Wavelength less than $10^{-3}$ nm
Longest wavelength; radio and TV broadcasting	Radio waves	Greater than $0.1$ m
$21$ cm hydrogen line from interstellar space	Microwaves	Long-distance communication, radar
Frequency $5\times10^{19}$ Hz	Gamma rays	Wavelength of order $10^{-11}$ to $10^{-12}$ m
$10^{-12}$ m $< \lambda < 10^{-8}$ m	X-rays	Crystal structure study, medical diagnosis
$10^{-11}$ m $< \lambda < 10^{-14}$ m	Gamma rays	Radiotherapy
$10^{-3}$ m $< \lambda < 10^{-1}$ m	Microwaves	Radar, microwave ovens
$10^{-4}$ m $< \lambda < 10^{-6}$ m	Infrared	Photography through fog and smoke

Points that carry the extra mark

Every band travels at $3\times10^{8}$ m s $^{-1}$ in vacuum, so two light waves of different wavelength have a speed ratio of $1:1$ .
Em waves carry no net charge and are undeflected by electric and magnetic fields.
Photon energy $E = h\nu$ grows from radio to gamma; the different photon-energy scales match the different sources — circuit currents (radio), molecular vibrations (infrared), outer-electron transitions (visible), inner-shell transitions (UV and X-rays), nuclear processes (gamma).
An em wave exerts radiation pressure on a surface because it carries momentum $p = U/c$ ; the pressure is $I/c$ for a fully absorbing surface and $2I/c$ for a fully reflecting one.
When asked to arrange wavelengths in ascending order for the standard trio: ultraviolet ( $10$ nm to $400$ nm) $<$ infrared ( $700$ nm to $1$ mm) $<$ microwaves ( $1$ mm to $0.1$ m).

Theory Questions Asked on This Subtopic

Real board questions. Answers are at the end of the page.

1
Name the electromagnetic radiation used for killing germs in water purifiers.
1 markCBSE Class 12 Physics — Very Short Answer · 2021C · Section A, 1 markAnswer 1 ↓
2
Write one use of the electromagnetic waves of frequency range from $10^{16}$ Hz to $10^{20}$ Hz.
1 markCBSE Class 12 Physics — Very Short Answer · 2020 · Section A, 1 markAnswer 2 ↓
3
Why are infrared radiations also referred to as heat waves? Write the names of radiations which lie next to infrared radiations in the electromagnetic spectrum on (a) shorter wavelength side, and (b) longer wavelength side.
1 markCBSE Class 12 Physics — Very Short Answer · 2020C · Section A, 1 markAnswer 3 ↓
4
Which part of the electromagnetic spectrum is used in RADAR? Give its frequency range.
1 markCBSE Class 12 Physics — Very Short Answer · 2019 · Section A, 1 markAnswer 4 ↓
5
Name the part of the electromagnetic spectrum which has the longest wavelength and write its one use.
1 markCBSE Class 12 Physics — Very Short Answer · 2019C · Section A, 1 markAnswer 5 ↓
6
The small ozone layer on top of the stratosphere is crucial for human survival. Why?
1 markCBSE Class 12 Physics — Very Short Answer · AI 2019, U · Section A, 1 markAnswer 6 ↓
7
Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.
1 markCBSE Class 12 Physics — Very Short Answer · 2018 · Section A, 1 markAnswer 7 ↓
8
Why are microwaves considered suitable for radar systems used in aircraft navigation?
1 markCBSE Class 12 Physics — Very Short Answer · Delhi 2016 · Section A, 1 markAnswer 8 ↓
9
To which part of the electromagnetic spectrum does a wave of frequency $5\times10^{19}$ Hz belong?
1 markCBSE Class 12 Physics — Very Short Answer · AI 2014, R · Section A, 1 markAnswer 9 ↓
10
Arrange the following electromagnetic waves in order of increasing frequency: $\gamma$ -rays, Microwaves, Infrared rays and Ultraviolet rays.
1 markCBSE Class 12 Physics — Very Short Answer · Foreign 2014, Ap · Section A, 1 markAnswer 10 ↓
11
Write any two characteristics of an electromagnetic wave. Why are microwaves used in radar systems?
2 marksCBSE Class 12 Physics — SA · 2023, An · Section B, 2 marksAnswer 11 ↓
12
Which of the following electromagnetic waves has (a) minimum wavelength, and (b) minimum frequency? Write one use of each of these two waves. Infrared waves, Microwaves, $\gamma$ -rays and X-rays
2 marksCBSE Class 12 Physics — SA · 2020 · Section B, 2 marksAnswer 12 ↓
13
Gamma rays and radio waves travel with the same velocity in free space. Distinguish between them in terms of their origin and their main application.
2 marksCBSE Class 12 Physics — SA · 2020, An · Section B, 2 marksAnswer 13 ↓
14
Why are infrared waves often called heat waves? Explain.
2 marksCBSE Class 12 Physics — SA · 2018 · Section B, 2 marksAnswer 14 ↓
15
Identify the electromagnetic waves whose wavelengths vary as (a) $10^{-12}$ m $<\lambda<10^{-8}$ m, (b) $10^{-3}$ m $<\lambda<10^{-1}$ m. Write one use for each.
2 marksCBSE Class 12 Physics — SA · AI 2017 · Section B, 2 marksAnswer 15 ↓
16
Identify the electromagnetic waves whose wavelengths vary as (a) $10^{-11}$ m $<\lambda<10^{-14}$ m, (b) $10^{-4}$ m $<\lambda<10^{-6}$ m. Write one use of each.
2 marksCBSE Class 12 Physics — SA · AI 2017, R · Section B, 2 marksAnswer 16 ↓
17
Name the types of em radiations which (i) are used in destroying cancer cells, (ii) cause tanning of the skin and (iii) maintain the earth's warmth. Write briefly a method of producing any one of these waves.
2 marksCBSE Class 12 Physics — SA · AI 2015C, Ev · Section B, 2 marksAnswer 17 ↓
18
Electromagnetic waves of wavelength $\lambda_1$ , $\lambda_2$ and $\lambda_3$ are used in radar systems, in water purifiers and in remote switches of TV, respectively. (i) Identify the electromagnetic waves, and (ii) Write one source of each of them.
3 marksCBSE Class 12 Physics — SA · Term II 2021-22 · Section C, 3 marksAnswer 18 ↓
19
Write the wavelength range and name of the electromagnetic waves which are used in (i) radar systems for aircraft navigation, and (ii) Earth satellites to observe the growth of crops.
3 marksCBSE Class 12 Physics — SA · 2020, R · Section C, 3 marksAnswer 19 ↓
20
Identify the part of the electromagnetic spectrum used in (i) radar and (ii) eye surgery. Write their frequency range.
3 marksCBSE Class 12 Physics — SA · Delhi 2019 · Section C, 3 marksAnswer 20 ↓
21
Identify the part of the electromagnetic spectrum which is: (a) suitable for radar system used in aircraft navigation, (b) produced by bombarding a metal target by high speed electrons.
3 marksCBSE Class 12 Physics — SA · AI 2016, R · Section C, 3 marksAnswer 21 ↓
22
(i) Which segment of electromagnetic waves has the highest frequency? How are these waves produced? Give one use of these waves. (ii) Which e.m. waves lie near the high frequency end of the visible part of the e.m. spectrum? Give its one use. In what way does this component of light have harmful effects on humans?
3 marksCBSE Class 12 Physics — SA · Foreign 2016, Ap · Section C, 3 marksAnswer 22 ↓
23
Name the parts of the electromagnetic spectrum which is (a) suitable for radar systems used in aircraft navigation, (b) used to treat muscular strain, (c) used as a diagnostic tool in medicine. Write in brief how these waves can be produced.
3 marksCBSE Class 12 Physics — SA · Delhi 2015, An · Section C, 3 marksAnswer 23 ↓
24
Answer the following: (a) Name the e.m. waves which are suitable for radar systems used in aircraft navigation. Write the range of frequency of these waves. (b) If the Earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now? Explain. (c) An e.m. wave exerts pressure on the surface on which it is incident. Justify.
3 marksCBSE Class 12 Physics — SA · Delhi 2014, Cr · Section C, 3 marksAnswer 24 ↓
25
Answer the following: (i) Show, by giving a simple example, how e.m. waves carry energy and momentum. (ii) How are microwaves produced? Why is it necessary in microwave ovens to select the frequency of microwaves to match the resonant frequency of water molecules? (iii) Write two important uses of infrared waves.
3 marksCBSE Class 12 Physics — SA · Delhi 2014C · Section C, 3 marksAnswer 25 ↓

Numericals

Every numerical set on this subtopic. Full working is at the end of the page.

26
Name the electromagnetic waves also known as 'heat waves'.
- (a)
  Radiowaves
- (b)
  Microwaves
- (c)
  X-rays
- (d)
  Infrared waves
1 markCBSE Class 12 Physics — Multiple Choice · 2023 · Section A, 1 markAnswer 26 ↓
27
Which one of the following electromagnetic radiation has the least wavelength?
- (a)
  Gamma rays
- (b)
  Microwaves
- (c)
  Visible light
- (d)
  X-rays
1 markCBSE Class 12 Physics — Multiple Choice · 2023, R · Section A, 1 markAnswer 27 ↓
28
A welder wears special glasses to protect his eyes mostly from the harmful effect of
- (a)
  very intense visible light
- (b)
  infrared radiation
- (c)
  ultraviolet rays
- (d)
  microwaves.
1 markCBSE Class 12 Physics — Multiple Choice · 2020, R · Section A, 1 markAnswer 28 ↓
29
Electromagnetic waves used as a diagnostic tool in medicine are
- (a)
  X-rays
- (b)
  ultraviolet rays
- (c)
  infrared radiation
- (d)
  ultrasonic waves.
1 markCBSE Class 12 Physics — Multiple Choice · 2020, R · Section A, 1 markAnswer 29 ↓

Answers & Solutions

Work the questions above first, then check yourself here. Each answer is set out the way it should be written in the paper.

1.↑ back to the question
Ultraviolet (UV) radiation is used for killing germs/bacteria in water purifiers.
2.↑ back to the question
This frequency range ( $10^{16}$ – $10^{20}$ Hz) corresponds to X-rays. Use: X-rays are used to detect fractures in bones (as a diagnostic tool in medicine) and to study the structure of crystals.
3.↑ back to the question
Infrared radiation, when incident on a substance, is readily absorbed and increases the internal (thermal) energy of the substance, raising its temperature — hence it is called heat radiation/heat waves. (a) On the shorter-wavelength side of infrared lies visible light. (b) On the longer-wavelength side of infrared lies microwaves.
4.↑ back to the question
Microwaves are used in RADAR systems (as they can be transmitted as beams in a specific direction and have short wavelengths suited to detecting the size of the reflecting object). Frequency range of microwaves: approximately $3\times10^8$ Hz to $3\times10^{11}$ Hz.
5.↑ back to the question
Radio waves have the longest wavelength in the electromagnetic spectrum (ranging roughly from about 1 mm to several kilometres). Use: radio waves are used for radio and television broadcasting/communication.
6.↑ back to the question
The ozone layer absorbs most of the harmful ultraviolet radiation present in sunlight, preventing it from reaching the Earth's surface. Without this shielding, UV radiation reaching the surface would penetrate living tissue and could cause serious harm, such as skin cancer and damage to eyes and the immune system — hence the ozone layer, though thin, is crucial for human (and other life) survival.
7.↑ back to the question
(a) Ultraviolet rays are used for water purification (to kill germs/bacteria). (b) Ultraviolet rays are also used in eye surgery (e.g. LASIK-type corneal procedures).
8.↑ back to the question
Microwaves have relatively short wavelengths, which allows them to be transmitted as narrow, well-directed beams (with less diffraction spreading) towards a target and reflected back; their short wavelength also gives good resolution for detecting the size and position of the object (such as an aircraft) — this makes them well suited to radar systems.
9.↑ back to the question
A frequency of $5\times10^{19}$ Hz is extremely high, corresponding to a wavelength on the order of $10^{-11}$ – $10^{-12}$ m. This lies in the gamma-ray region of the electromagnetic spectrum.
10.↑ back to the question
In order of increasing frequency: Microwaves $<$ Infrared rays $<$ Ultraviolet rays $<$ $\gamma$ -rays.
11.↑ back to the question
Two characteristics of an electromagnetic wave: (i) it does not carry any net electric charge, and is not deflected by electric or magnetic fields; (ii) in vacuum, it travels with the same speed $c=3\times10^8$ m/s, regardless of wavelength. Microwaves have a short wavelength, which lets them be transmitted as a narrow, directed beam with good resolving power for locating objects, and they can also penetrate through clouds/haze — this makes them well suited to radar systems.
12.↑ back to the question
(a) $\gamma$ -rays have the minimum wavelength among the four listed. Use: $\gamma$ -rays are used in the treatment of cancer (radiotherapy). (b) Microwaves have the minimum frequency among the four listed. Use: microwaves are used in radar systems (aircraft navigation) and in microwave ovens for cooking.
13.↑ back to the question
Origin: $\gamma$ -rays are of nuclear origin — they are produced in the disintegration (radioactive decay) of atomic nuclei and in the decay of certain subatomic particles. Radio waves are produced by accelerated motion of charges in conducting wires, e.g. in oscillating LC circuits/antennas. Application: $\gamma$ -rays, being highly energetic and penetrating, are used in the treatment of cancer/tumours (radiotherapy) and in sterilization. Radio waves are used for radio, television and satellite communication.
14.↑ back to the question
Infrared radiation, on falling on a substance, is readily absorbed by the molecules of the substance (water, CO $_2$ , NH $_3$ and similar molecules absorb it particularly well), which increases their thermal (vibrational/rotational) motion. This absorbed energy raises the internal energy and hence the temperature of the substance — for this reason, infrared radiation is often called heat radiation, or heat waves.
15.↑ back to the question
(a) This wavelength range corresponds to X-rays. Use: X-rays are used to study the structure of crystals and atomic arrangements (X-ray diffraction), and as a diagnostic tool in medicine. (b) This wavelength range corresponds to microwaves. Use: microwaves are used in radar applications (aircraft navigation) and in microwave ovens.
16.↑ back to the question
(a) This wavelength range corresponds to $\gamma$ -rays. Use: $\gamma$ -rays are used in radiotherapy to treat cancers/tumours. (b) This wavelength range corresponds to infrared waves. Use: infrared waves are used to take photographs during conditions of fog/smoke, since they are scattered much less than visible light by fine particles.
17.↑ back to the question
(i) $\gamma$ -rays are used in destroying cancer cells (radiotherapy). (ii) Ultraviolet rays cause tanning (and, in excess, burning) of the skin. (iii) Infrared radiations help maintain the Earth's warmth (the greenhouse effect: they are absorbed and re-radiated by atmospheric gases, trapping heat near the surface). Method of production: infrared waves are produced by hot bodies and molecules (any object above absolute zero emits infrared radiation because of the vibrational and rotational motion of its molecules/atoms).
18.↑ back to the question
(i) $\lambda_1$ (radar systems) $\to$ Microwaves, with $1\text{ mm}<\lambda_1<0.1\text{ m}$ . $\lambda_2$ (water purifiers) $\to$ Ultraviolet waves, with $10\text{ nm}<\lambda_2<400\text{ nm}$ . $\lambda_3$ (TV remote switches) $\to$ Infrared waves, with $700\text{ nm}<\lambda_3<1\text{ mm}$ . (ii) Source of $\lambda_1$ (microwaves): special vacuum tubes such as klystrons, magnetrons or Gunn diodes. Source of $\lambda_2$ (ultraviolet): very hot bodies such as the Sun, and electron transitions in the inner shells of heavy atoms. Source of $\lambda_3$ (infrared): vibrations of atoms and molecules (any warm body).
19.↑ back to the question
(i) Radar systems for aircraft navigation use microwaves, with wavelength range approximately $10^{-3}$ m to $10^{-1}$ m. (ii) Earth satellites used to observe the growth of crops (remote sensing) use infrared waves, with wavelength range approximately $7.5\times10^{-7}$ m to $10^{-3}$ m.
20.↑ back to the question
(i) Radar uses microwaves, with frequency range approximately $3\times10^8$ Hz to $3\times10^{11}$ Hz. (ii) Eye surgery uses ultraviolet rays, with frequency range approximately $8\times10^{14}$ Hz to $8\times10^{16}$ Hz.
21.↑ back to the question
(a) Microwaves are suitable for radar systems used in aircraft navigation (short wavelength gives a well-directed beam and good resolving power). (b) X-rays are produced by bombarding a metal target of high atomic number with high-speed electrons (the electrons are suddenly decelerated on striking the target, emitting X-rays — Bremsstrahlung).
22.↑ back to the question
(i) $\gamma$ -rays have the highest frequency in the electromagnetic spectrum. They are of nuclear origin, produced in the disintegration (radioactive decay) of atomic nuclei and in the decay of certain subatomic particles. Use: $\gamma$ -rays are used in the treatment of cancer/tumours. (ii) Ultraviolet rays lie near the high-frequency end of the visible part of the spectrum. Use: UV rays are used to preserve foodstuff (by killing microorganisms). Harmful effects: excessive exposure to UV causes premature ageing of the skin, suppresses the immune system, and can damage the eyes and cause skin cancer.
23.↑ back to the question
(a) Microwaves are suitable for radar systems used in aircraft navigation; they are produced by special vacuum tubes such as klystrons, magnetrons, or Gunn diodes. (b) Infrared waves are used to treat muscular strain (pain relief through heating); they are produced by hot bodies and molecules. (c) X-rays are used as a diagnostic tool in medicine; they are produced when high-energy electrons are suddenly stopped upon striking a metal target of high atomic number.
24.↑ back to the question
(a) Microwaves are suitable for radar systems used in aircraft navigation; their frequency range is approximately $10^8$ Hz to $10^{11}$ Hz. (b) If the Earth had no atmosphere, its average surface temperature would be lower than it is now. This is because the atmosphere (via greenhouse gases) absorbs and re-radiates infrared radiation emitted by the Earth's surface back towards it, trapping heat (the greenhouse effect); without this atmosphere, that trapped infrared energy would simply escape into space, so the surface would be cooler. (c) An electromagnetic wave carries momentum $p=U/c$ , where $U$ is the energy delivered. When the wave is incident on a surface and is absorbed (or reflected), this momentum is transferred to the surface. By Newton's second law, a rate of change of momentum corresponds to a force; hence the wave exerts a (radiation) pressure, $P=I/c$ for a fully absorbing surface (and $2I/c$ for a fully reflecting one, where $I$ is the intensity), on the surface.
25.↑ back to the question
(i) Consider a plane surface placed perpendicular to the direction of propagation of an em wave, with some electric charge resting on it. As the wave's oscillating electric field passes over the charge, it sets the charge into oscillatory motion — this shows the wave delivers energy to the charge (which now has kinetic energy) and, because the charge's oscillating motion also interacts with the wave's magnetic field, exerts a force on it in the direction of propagation, showing the wave also delivers momentum. A material surface heating up and experiencing radiation pressure under sunlight is a familiar large-scale example of the same effect. (ii) Microwaves are produced by special vacuum tubes such as klystrons, magnetrons, or Gunn diodes. In a microwave oven, the frequency of the microwaves is chosen to match the resonant (natural) frequency of vibration of water molecules in food, so that the molecules absorb the microwave energy most efficiently, converting it rapidly into kinetic (thermal) energy and heating the food effectively. (iii) Two uses of infrared waves: (a) used in night-vision/military surveillance devices, since infrared radiation can pass through haze, fog and mist better than visible light; (b) used in remote-control switches of household electrical appliances (such as TV remotes).
26.Answer: (D)↑ back to the question
Infrared waves are called heat waves because they raise the temperature of objects on which they fall (they are readily absorbed by most materials, including water, and increase the internal/thermal energy of the absorber). Hence option (d).
27.Answer: (A)↑ back to the question
In the electromagnetic spectrum, gamma rays have the shortest wavelength (and correspondingly the highest frequency) among the options listed. Hence option (a).
28.Answer: (C)↑ back to the question
The welding arc emits intense ultraviolet radiation, which can damage the eyes (and skin); the welder's special glasses are designed mainly to block this UV radiation. Hence option (c).
29.Answer: (A)↑ back to the question
X-rays are widely used as a diagnostic tool in medicine, e.g. to detect fractures in bones and to image dense internal structures, because of their high penetrating power through soft tissue. Hence option (a).