PhysicsCBSE Class 12 · Electromagnetic Waves

Electromagnetic Spectrum Class 12: From Radio Waves to Gamma Rays

Q: What is the electromagnetic spectrum?

It is the complete range of electromagnetic waves arranged by wavelength or frequency, from long radio waves to very short gamma rays. All are the same kind of wave and travel at c in vacuum.

Q: Are the boundaries between regions exact?

No. Regions overlap and are named mainly by how the waves are produced or detected; the wavelength ranges are approximate.

Q: Why are infrared waves called heat waves?

Water and many other molecules absorb infrared readily. The absorbed energy increases their thermal motion, so the material warms up.

Q: Why is UV radiation harmful?

UV photons carry enough energy to damage molecules in skin and eye cells. Small doses cause tanning; large doses can cause burns and long-term damage.

Q: Which EM waves are used in mobile phones?

Radio waves in the ultra-high-frequency (UHF) range, near the boundary with microwaves.

Updated 15 October 2026 · Fact-checked

The electromagnetic spectrum is the full range of EM waves arranged by wavelength or frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays, in order of decreasing wavelength. All travel at c in vacuum. They differ in how they are produced and detected and in their uses, from communication to medicine.

Moving from radio to gamma rays, wavelength shrinks and frequency rises, but every wave still travels at c in vacuum.

Illustration drawn for QuestionBuddy.

Understand Electromagnetic Spectrum

Your phone, a microwave oven, a TV remote, sunlight, a hospital X-ray and a nuclear reactor all involve the same kind of wave. They differ only in wavelength (and frequency), yet that difference changes everything about how they are made, detected and used.

The spectrum. Arranged in order of decreasing wavelength (increasing frequency and photon energy): radio waves → microwaves → infrared → visible → ultraviolet → X-rays → gamma rays. There are no sharp boundaries; neighbouring regions overlap, and the names reflect mainly how the waves are produced or detected. All of them travel at $c = 3 \times 1 0^{8}$ m/s in vacuum, so $c = ν λ$ links frequency and wavelength.

Each region in brief (approximate wavelength ranges).

Radio waves (more than about 0.1 m): produced by accelerated charges in conducting wires and aerials; received by aerials. Used in radio and TV broadcasting and mobile communication. AM radio uses frequencies around 0.5 to 1.7 MHz; FM radio about 88 to 108 MHz.
Microwaves (about 0.1 m to 1 mm, gigahertz frequencies): produced by special vacuum tubes such as klystrons and magnetrons. Their short wavelength suits radar (for aircraft navigation and speed guns). Microwave ovens use a frequency at which water molecules absorb energy well, heating food that contains water.
Infrared (about 1 mm to 700 nm): produced by hot bodies and vibrating molecules; detected by thermopiles and bolometers. Called heat waves because water and many molecules absorb them readily and warm up. Uses: physical therapy lamps, remote controls (from LEDs), night vision and satellite crop monitoring. Infrared re-radiated by the Earth's surface is trapped by greenhouse gases, keeping the planet warm (the greenhouse effect).
Visible light (about 700 nm to 400 nm, roughly $4 \times 1 0^{14}$ to $7 \times 1 0^{14}$ Hz): emitted when electrons in atoms drop to lower energy levels; detected by the eye, photocells and film. Our sense of sight depends on it.
Ultraviolet (about 400 nm to 1 nm): produced by special lamps and very hot bodies such as the Sun. Most solar UV is absorbed by the ozone layer. Large doses are harmful; small doses cause tanning. Ordinary glass absorbs UV. Uses: killing germs in water purifiers, and precise eye surgery (LASIK), since its short wavelength can be focused into narrow beams.
X-rays (about 1 nm to $1 0^{- 3}$ nm): produced when high-energy electrons strike a metal target; detected by photographic film and Geiger tubes. Used in medical diagnosis (they pass through soft tissue but are absorbed more by bone) and in treating cancer; overexposure damages living tissue.
Gamma rays (less than about $1 0^{- 3}$ nm): emitted in nuclear reactions and radioactive decay. Highest energy; used in medicine to destroy cancer cells.

Energy increases along the spectrum. The energy of each photon is $E = h ν = \frac{h c}{λ}$ (with $h c \approx 1240$ eV nm). Visible photons carry about 2 to 3 eV, X-ray photons thousands of eV. That is why UV, X-rays and gamma rays can ionise atoms and damage cells, while radio waves cannot.

Electromagnetic Spectrum: formulas and key facts

Wave relation (all EM waves): $c = ν λ = 3 \times 1 0^{8} m/s$; Use to switch between frequency and wavelength.
Order by decreasing wavelength: $radio > micro > IR > visible > UV > X-ray > γ$; Frequency and photon energy increase in the same order.
Visible range: $λ \approx 400 - 700 nm, ν \approx 4 - 7.5 \times 1 0^{14} Hz$; Violet at the short end, red at the long end.
Photon energy: $E = h ν = \frac{h c}{λ}, E (eV) \approx \frac{1240}{λ ( nm )}$; Explains why short-wavelength radiation is more damaging.

How to solve Electromagnetic Spectrum questions

Use this method for spectrum questions: identification, ordering, uses and simple calculations.

1Convert any given frequency to wavelength (or the reverse) with $λ = \frac{c}{ν}$ .
2Place the wavelength in the right region using the approximate ranges: radio > 0.1 m, microwave 0.1 m to 1 mm, IR 1 mm to 700 nm, visible 700 to 400 nm, UV 400 to 1 nm, X-ray 1 to 10⁻³ nm, gamma below that.
3For uses, give one clear use per region with a one-line reason (for example, short wavelength for radar).
4For production and detection, name the source and detector for the region asked.
5For energy comparisons, use $E = \frac{h c}{λ}$ ; a shorter wavelength means a more energetic photon.
6When ordering, state whether you are arranging by increasing or decreasing wavelength or frequency.

Electromagnetic Spectrum in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Microwaves and water

Suppose a microwave oven operates at 2.45 GHz.

(a) Find the wavelength of its microwaves. (b) Why does food containing water heat up, while a dry ceramic plate stays relatively cool? (c) In which region of the spectrum are these waves?

Try it first, then show the solutionSolution

$λ = \frac{c}{ν} = \frac{3 \times 1 0 ^{8}}{2.45 \times 1 0 ^{9}} \approx 0.122$ m, about 12 cm.
The frequency is chosen so that water molecules absorb the wave's energy efficiently and move faster, raising the temperature; a dry plate has little water to absorb it.
At about 12 cm the wavelength sits just above 0.1 m, where the radio and microwave regions meet; GHz waves made by magnetrons are called microwaves, and the boundary is not sharp.

Answer: (a) about 12 cm; (b) water molecules absorb this frequency and heat up; (c) the microwave region (near its long-wavelength edge).

Memory hook: Microwave ovens heat the water, not the plate.

Infrared in remote controls

Imagine a TV remote whose LED emits radiation of wavelength 940 nm.

(a) Find its frequency and photon energy. (b) Why can't you see it? (c) Name a simple way to check whether the remote is working.

Try it first, then show the solutionSolution

$ν = \frac{3 \times 1 0 ^{8}}{940 \times 1 0 ^{- 9}} \approx 3.19 \times 1 0^{14}$ Hz; $E \approx \frac{1240}{940} \approx 1.32$ eV.
940 nm is longer than 700 nm, so it is infrared, beyond the red end of what the eye detects.
Many phone cameras respond to near-infrared: point the remote at the camera and press a button, and a flash may be seen on the screen.

Answer: (a) about 3.19 × 10¹⁴ Hz and 1.32 eV; (b) it is infrared, outside the visible range; (c) view it through a phone camera.

Memory hook: Beyond red is infrared: invisible but useful.

Tricks to remember Electromagnetic Spectrum

Short memory aids. Say each one aloud once, then use it on a question the same day.

Raging Martians Invaded Venus Using X-ray Guns
Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma: decreasing wavelength.
Use it when: Getting the order right.
Shorter means stronger
Shorter wavelength → higher frequency → more energetic photons → more damaging.
Use it when: Explaining hazards of UV, X-rays and gamma rays.
1240 over nanometres
Photon energy in eV ≈ 1240/λ (nm).
Use it when: Quick energy estimates.
One use, one reason
Radar: short λ (microwave). Tanning: UV. Bones: X-ray. Heat: IR.
Use it when: Uses questions.

Electromagnetic Spectrum: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Ordering the spectrum · 2 marks · Very short answerBoard level

Arrange the following in increasing order of wavelength: X-rays, microwaves, ultraviolet, gamma rays, infrared, visible light. Which has the highest frequency?

Try it first, then show the solutionSolution

Increasing wavelength: gamma rays < X-rays < ultraviolet < visible < infrared < microwaves. (1)
Highest frequency: gamma rays (shortest wavelength). (1)

Answer: γ < X < UV < visible < IR < microwave; gamma rays have the highest frequency.

Example 2: Production and uses · 3 marks · Short answerBoard level

For each of (a) microwaves, (b) infrared, (c) ultraviolet and (d) X-rays, state how it is produced and give one use.

Try it first, then show the solutionSolution

(a) Microwaves: special vacuum tubes such as klystrons and magnetrons; used in radar and microwave ovens. (¾)
(b) Infrared: hot bodies and vibrating molecules; used in physiotherapy lamps and remote controls. (¾)
(c) Ultraviolet: special lamps and very hot bodies like the Sun; used to kill germs in water purifiers. (¾)
(d) X-rays: high-energy electrons striking a metal target; used to detect bone fractures. (¾)

Answer: Microwaves (klystron/magnetron; radar), infrared (hot bodies; physiotherapy), UV (special lamps, Sun; sterilisation), X-rays (electrons on a metal target; medical imaging).

Example 3: Identify the radiation · 3 marks · Short answerHarder board level

Identify the region of the spectrum for waves of (a) 10 GHz, (b) $5 \times 1 0^{14}$ Hz, (c) $3 \times 1 0^{18}$ Hz and (d) 1 MHz, and give a use of each.

Try it first, then show the solutionSolution

(a) $λ = 3$ cm: microwaves; radar. (½ + ¼)
(b) $λ = 600$ nm: visible (orange-red); vision and optical instruments. (½ + ¼)
(c) $λ = 0.1$ nm: X-rays; medical diagnosis. (½ + ¼)
(d) $λ = 300$ m: radio waves; AM broadcasting. (½ + ¼)

Answer: (a) microwaves; (b) visible; (c) X-rays; (d) radio waves.

Example 4: Photon energies across the spectrum · 3 marks · Short answerHarder board level

Compare the photon energy of X-rays of wavelength 0.1 nm with that of visible light of wavelength 500 nm. Use $h c \approx 1240$ eV nm. Explain why X-rays are hazardous but visible light is not.

Try it first, then show the solutionSolution

X-ray: $E = \frac{1240}{0.1} = 12400$ eV ≈ 12.4 keV. (1)
Visible: $E = \frac{1240}{500} = 2.48$ eV; ratio = 5000. (1)
X-ray photons carry enough energy to ionise atoms and break bonds in cells; visible photons (a few eV) generally cannot. (1)

Answer: 12.4 keV against 2.48 eV, 5000 times more; X-rays can ionise atoms and damage tissue.

Example 5: Reasons behind uses · 5 marks · Long answerHOTS / competency

Give reasons:
(a) Microwaves are used in radar. (1)
(b) Infrared waves are called heat waves. (1)
(c) One does not get sunburnt through ordinary glass windows. (1)
(d) X-rays are used to detect fractures. (1)
(e) The ozone layer is important for life on Earth. (1)

Try it first, then show the solutionSolution

(a) Their short wavelength allows them to be sent as narrow beams and to reflect well from objects such as aircraft, giving accurate direction and distance. (1)
(b) Water and many other molecules absorb infrared readily; their thermal motion increases, so objects absorbing IR heat up. (1)
(c) Ordinary glass absorbs ultraviolet, so little UV passes through. (1)
(d) X-rays pass easily through soft tissue but are absorbed more by bone, so bones and breaks show up clearly on film. (1)
(e) It absorbs most of the Sun's harmful ultraviolet radiation, which in large amounts damages living cells. (1)

Answer: Short λ for radar; IR absorbed and heats; glass absorbs UV; bone absorbs X-rays more than tissue; ozone absorbs harmful UV.

Example 6: Earth's energy balance · 4 marks · Case-basedHarder board level

Read the passage and answer.

Sunlight, mostly visible, passes through the atmosphere and warms the ground. The warm ground re-emits energy at longer wavelengths, much of which is absorbed by carbon dioxide and water vapour in the atmosphere.
(a) In which region of the spectrum does the ground re-emit? (1)
(b) Name this warming effect. (1)
(c) Why does visible light pass through the atmosphere while the re-emitted radiation is trapped? (1)
(d) Which radiation from the Sun is largely stopped high in the atmosphere, and by what? (1)

Try it first, then show the solutionSolution

(a) Infrared. (1)
(b) The greenhouse effect. (1)
(c) Greenhouse gases such as CO₂ and water vapour absorb infrared strongly but let visible light through. (1)
(d) Most ultraviolet radiation, absorbed by the ozone layer. (1)

Answer: (a) infrared; (b) greenhouse effect; (c) CO₂ and water vapour absorb IR but not visible light; (d) UV, absorbed by ozone.

Where students lose marks in Electromagnetic Spectrum

The slips examiners see most often, and what to write instead.

Mistake: Getting the order reversed.
Not stating whether ordering by wavelength or frequency.
Do this: Write 'increasing wavelength' or 'increasing frequency' before the list.
Usually costs: 1 mark.
Mistake: Saying different EM waves travel at different speeds in vacuum.
Confusing them with sound or matter waves.
Do this: All EM waves travel at c in vacuum.
Usually costs: 1 mark.
Mistake: Giving a use without a reason when 'why' is asked.
Listing facts only.
Do this: Pair each use with the property that makes it suitable.
Usually costs: ½ mark per part.
Mistake: Mixing up microwaves and radio waves, or IR and UV.
Similar names.
Do this: Infra = below red (longer λ); ultra = beyond violet (shorter λ).
Usually costs: 1 mark.

Exam tips

Learn the order and one use, one source and one detector for each region; this covers most 1- to 3-mark items.
Spectrum questions appeared in board papers of every year from 2023 to 2026 (for example 2023 55/5/1 Q21, 2024 55/1/1 Q26, 2025 55/1/1 Q7 and 2026 55/1/1 Q7).
Show the conversion λ = c/ν when identifying a region from a frequency.
Use 'elementary facts' only: the syllabus does not ask for detailed production mechanisms.
Remember that the boundaries are not sharp; mention 'approximately' with ranges.

Practice questions on Electromagnetic Spectrum

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of these has the shortest wavelength?
(a) microwaves (b) ultraviolet (c) X-rays (d) gamma rays
Hint
End of the spectrum.
Answer
(d) gamma rays.
Q2 · 2 marks · Very short answerBoard level
Name the EM waves used (a) in radar, (b) to kill germs in water purifiers, (c) in TV remote controls and (d) to detect bone fractures.
Hint
One region each.
Answer
(a) Microwaves; (b) ultraviolet; (c) infrared; (d) X-rays.
Q3 · 1 mark · Very short answerHarder board level
Find the frequency of radiation of wavelength 0.5 nm and name its region.
Hint
ν = c/λ.
Answer
$ν = \frac{3 \times 1 0 ^{8}}{5 \times 1 0 ^{- 10}} = 6 \times 1 0^{17}$ Hz: X-rays.
Q4 · 1 mark · Very short answerHarder board level
Find the frequency range of visible light (400 nm to 700 nm).
Hint
c/λ at both ends.
Answer
From $\frac{3 \times 1 0 ^{8}}{7 \times 1 0 ^{- 7}} \approx 4.3 \times 1 0^{14}$ Hz (red) to $\frac{3 \times 1 0 ^{8}}{4 \times 1 0 ^{- 7}} = 7.5 \times 1 0^{14}$ Hz (violet).
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): X-rays are used in medical diagnosis.
Reason (R): X-rays pass through soft tissue easily but are absorbed more strongly by bones.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Contrast on film.
Answer
(a).
Q6 · 2 marks · Very short answerHOTS / competency
Arrange microwaves, ultraviolet, infrared and gamma rays in increasing order of photon energy and justify the order.
Hint
E = hc/λ.
Answer
Microwaves < infrared < ultraviolet < gamma rays. Photon energy hc/λ rises as wavelength falls, and the wavelengths decrease in this order.
Q7 · 2 marks · Very short answerHOTS / competency
Welders wear goggles with special glass windows, and doctors limit the number of X-ray images a patient has. Explain both precautions in terms of the radiation involved.
Hint
UV from welding arcs; energy of X-ray photons.
Answer
Welding arcs emit a lot of ultraviolet, which can damage the eyes; the special glass absorbs UV. X-ray photons are energetic enough to ionise atoms in tissue, so repeated exposure increases the risk of cell damage; images are limited to what is necessary.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.EM waves in order of decreasing wavelength?Show answer
Radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
2.Wavelength range of visible light?Show answer
About 400 nm to 700 nm.
3.Which waves are called heat waves?Show answer
Infrared.
4.Which layer absorbs most solar UV?Show answer
The ozone layer.
5.EM waves used in radar?Show answer
Microwaves.
6.How are X-rays produced?Show answer
By high-energy electrons striking a metal target.

Electromagnetic Spectrum: frequently asked questions

What is the electromagnetic spectrum?

It is the complete range of electromagnetic waves arranged by wavelength or frequency, from long radio waves to very short gamma rays. All are the same kind of wave and travel at c in vacuum.

Are the boundaries between regions exact?

No. Regions overlap and are named mainly by how the waves are produced or detected; the wavelength ranges are approximate.

Why are infrared waves called heat waves?

Water and many other molecules absorb infrared readily. The absorbed energy increases their thermal motion, so the material warms up.

Why is UV radiation harmful?

UV photons carry enough energy to damage molecules in skin and eye cells. Small doses cause tanning; large doses can cause burns and long-term damage.

Which EM waves are used in mobile phones?

Radio waves in the ultra-high-frequency (UHF) range, near the boundary with microwaves.