PhysicsISC Class 12 · Electromagnetic Waves

The Electromagnetic Spectrum: Regions, Wavelengths, Sources, Detectors and Uses

Q: What is the difference between radio waves and microwaves?

Only wavelength. Radio waves are longer than about 0.1 m; microwaves run from 0.1 m to 1 mm, are made by magnetrons or klystrons, and are used in radar, satellite links and ovens.

Q: Do all electromagnetic waves travel at the same speed?

Yes in vacuum: 3 \times 10^8 m/s. In a medium the speed depends on the medium and the wavelength.

Q: Why are UV, X-rays and gamma rays harmful?

They have high frequency, so each photon carries enough energy to ionise atoms and damage living cells.

Q: What does the ISC 2027 syllabus need on the spectrum?

A qualitative account of all regions from radio to gamma: production, detection, other properties and uses, and the approximate ranges of λ or f, or at least their correct order.

Q: How has the spectrum been asked recently?

ISC 2024 Q5 (2 marks) asked the radiation of 10^{11} Hz and the speed of radio waves; ISC 2025 Q7 (2 marks) asked the wave from a satellite to a dish and the frequency for λ = 6 nm.

Updated 15 October 2026 · Fact-checked

The electromagnetic spectrum is the full range of electromagnetic waves arranged by wavelength or frequency. In order of decreasing wavelength (increasing frequency) it runs: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All travel in vacuum at $c = 3 \times 10^8$ m/s, so $c = f\lambda$ links frequency and wavelength.

Going from radio waves down to gamma rays, wavelength shrinks and frequency grows, because c = fλ stays fixed.

Illustration drawn for QuestionBuddy.

Understand The Electromagnetic Spectrum

One family, many names. Radio waves, light and gamma rays are all electromagnetic waves: oscillating electric and magnetic fields travelling at $c = 3 \times 1 0^{8}$ m/s in vacuum. They differ only in wavelength λ (and so frequency $f = \frac{c}{λ}$ ). The names come from how each band is produced and detected; the boundaries are not sharp and neighbouring bands overlap.

The regions (approximate wavelength ranges, from NCERT Table 8.1), from longest wavelength to shortest:

Radio waves, λ > 0.1 m. *Source*: accelerated electrons in antennas (oscillating circuits). *Detection*: receiving aerials. *Uses*: AM and FM broadcasting, television, mobile phones (UHF band).
Microwaves, 0.1 m to 1 mm. *Source*: klystron, magnetron and Gunn diodes. *Detection*: point-contact diodes. *Uses*: radar, satellite communication, microwave ovens (water molecules absorb energy at these frequencies).
Infrared (IR), 1 mm to 700 nm. *Source*: hot bodies; vibrations of atoms and molecules. *Detection*: thermopile, bolometer, IR film. *Uses*: remote controls, night vision, physiotherapy heat lamps, keeping the Earth warm (greenhouse effect).
Visible light, 700 nm (red) to 400 nm (violet). *Source*: electrons in atoms dropping between energy levels. *Detection*: eye, photocells, film. *Use*: vision, photography.
Ultraviolet (UV), 400 nm to 1 nm. *Source*: the Sun, arc lamps, very hot bodies; inner-shell electron transitions. *Detection*: photocells, film. *Uses*: killing germs (sterilising water), detecting forged notes by fluorescence. Absorbed by the ozone layer.
X-rays, 1 nm to $1 0^{- 3}$ nm. *Source*: X-ray tubes (fast electrons striking a metal target). *Detection*: photographic film, Geiger tube, ionisation chamber. *Uses*: imaging bones, airport baggage scans, studying crystal structure.
Gamma rays, below about $1 0^{- 3}$ nm. *Source*: radioactive nuclei and nuclear reactions. *Detection*: Geiger tube, ionisation chamber, film. *Uses*: cancer treatment, sterilising medical equipment.

Order to remember. Increasing frequency (and photon energy): radio < microwave < IR < visible < UV < X-ray < gamma. Wavelength runs the opposite way. As frequency increases, penetrating and ionising power increase, which is why UV, X-rays and gamma rays need care.

Common features of all regions. All are transverse (E and B perpendicular to each other and to the direction of travel), all travel at c in vacuum, none needs a medium, and none is deflected by electric or magnetic fields because they carry no charge.

The numerical link. $c = f λ$ , with λ in metres and f in hertz. Convert first: 1 nm = $1 0^{- 9}$ m, 1 µm = $1 0^{- 6}$ m, 1 MHz = $1 0^{6}$ Hz, 1 GHz = $1 0^{9}$ Hz. In ISC 2024 (Q5) candidates had to name the radiation of frequency $1 0^{11}$ Hz (λ = 3 mm: microwaves) and state the speed of radio waves in vacuum.

The Electromagnetic Spectrum: formulas and key facts

Wave relation: $c = f λ$; c = 3 × 10⁸ m/s in vacuum.
Frequency: $f = \frac{c}{λ}$; λ in metres.
Wavelength: $λ = \frac{c}{f}$; f in hertz.
Speed of every region: $c = \frac{1}{μ _{0} ε _{0}} \approx 3 \times 1 0^{8} m/s$; Same for all wavelengths in vacuum.

How to solve The Electromagnetic Spectrum questions

Spectrum questions either name a region or calculate f or λ; most need both.

1Convert the given value to SI units: λ in m, f in Hz.
2Use $f = \frac{c}{λ}$ or $λ = \frac{c}{f}$ with $c = 3 \times 1 0^{8}$ m/s.
3Compare λ with the ranges: > 0.1 m radio; 0.1 m to 1 mm microwave; 1 mm to 700 nm IR; 700 to 400 nm visible; 400 nm to 1 nm UV; 1 nm to $1 0^{- 3}$ nm X-ray; shorter gamma.
4Name the region and, if asked, give one source, one detector and one use.
5For "arrange" questions, write the full order once and pick out the ones asked.

The 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.

Sunburn and ultraviolet

Suppose you spend an afternoon at the beach and the Sun's ultraviolet of wavelength about 300 nm reaches your skin.

Find its frequency and explain why this burns the skin while the infrared you also receive only warms it.

Try it first, then show the solutionSolution

$f = \frac{3 \times 1 0 ^{8}}{300 \times 1 0 ^{- 9}} = 1 0^{15}$ Hz: ultraviolet.
UV has a higher frequency than IR, so each photon carries more energy; it can break chemical bonds in skin cells, while IR mainly makes molecules vibrate (heat).

Answer: $1 0^{15}$ Hz; higher-frequency UV damages cells, IR just heats.

Memory hook: Higher up the spectrum, harder the hit.

Weather radar

Imagine a weather radar at an airport that sends out pulses at 10 GHz and times the echoes from rain clouds.

Find the wavelength and name the region.

Try it first, then show the solutionSolution

$λ = \frac{3 \times 1 0 ^{8}}{10 \times 1 0 ^{9}} = 0.03$ m = 3 cm.
3 cm lies between 1 mm and 0.1 m: microwaves.

Answer: 3 cm; microwaves, which are short enough to form narrow beams and reflect from raindrops.

Memory hook: Radar rhymes with microwave range: centimetres.

Tricks to remember The 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: increasing frequency.
Use it when: Order of regions.
Up in f, down in λ
$f λ = c$ is fixed, so one goes up as the other goes down.
Use it when: Ordering by wavelength.
Visible is 400 to 700
Violet 400 nm, red 700 nm: "VR" from 4 to 7.
Use it when: Boundaries of IR and UV.
Source by energy
Antennas → radio; hot bodies → IR; atoms → light and UV; electrons hitting metal → X-rays; nuclei → gamma.
Use it when: Source questions.

The Electromagnetic Spectrum: solved examples

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

Example 1: Shortest wavelength · 1 mark · MCQBoard level

Which of these has the shortest wavelength?
(a) Microwaves
(b) Visible light
(c) Ultraviolet
(d) Infrared

Try it first, then show the solutionSolution

In order of decreasing wavelength: microwaves, infrared, visible, ultraviolet. UV is shortest of these.

Answer: (c).

Example 2: Naming a region from wavelength · 2 marks · Short answerBoard level

Electromagnetic radiation has a wavelength of 0.2 nm in vacuum. Find its frequency and name the region of the spectrum.

Try it first, then show the solutionSolution

$f = \frac{3 \times 1 0 ^{8}}{0.2 \times 1 0 ^{- 9}} = 1.5 \times 1 0^{18}$ Hz. (1)
0.2 nm lies between 1 nm and $1 0^{- 3}$ nm: X-rays. (1)

Answer: $1.5 \times 1 0^{18}$ Hz; X-rays.

Example 3: Source and detector · 3 marks · Short answerHarder board level

For (a) radio waves, (b) visible light and (c) gamma rays, state one source and one detector.

Try it first, then show the solutionSolution

(a) Radio: oscillating electrons in a transmitting antenna; detected by a receiving aerial. (1)
(b) Visible: electrons in atoms falling to lower energy levels (as in a lamp); detected by the eye or a photocell. (1)
(c) Gamma: radioactive nuclei; detected by a Geiger tube or ionisation chamber. (1)

Answer: As listed: antenna/aerial; atomic transitions/eye; nuclei/Geiger tube.

Example 4: Phone call versus Wi-Fi · 2 marks · Short answerHarder board level

A mobile phone uses signals of 900 MHz and a Wi-Fi router uses 5 GHz. Find each wavelength and name the region.

Try it first, then show the solutionSolution

900 MHz: $λ = \frac{3 \times 1 0 ^{8}}{9 \times 1 0 ^{8}} \approx 0.33$ m, more than 0.1 m: radio waves (UHF band). (1)
5 GHz: $λ = \frac{3 \times 1 0 ^{8}}{5 \times 1 0 ^{9}} = 0.06$ m = 6 cm: microwaves. (1)

Answer: ≈ 0.33 m, radio; 6 cm, microwaves.

Example 5: Why X-rays and not visible light · 2 marks · Short answerHOTS / competency

A crystal has atoms about 0.1 nm apart. Explain why X-rays, not visible light of 600 nm, are used to study its structure. By what factor is the visible wavelength larger?

Try it first, then show the solutionSolution

Diffraction is noticeable only when the wavelength is comparable to the spacing; X-rays (≈ 0.1 nm) match the atomic spacing. (1)
$\frac{600 nm}{0.1 nm} = 6000$ : visible light is far too long to be diffracted by the atomic layers. (1)

Answer: X-rays have λ comparable to atomic spacing; visible light is 6000 times longer.

Example 6: Inside a smartphone · 4 marks · Case-basedHOTS / competency

Suppose a smartphone uses: a GPS receiver at 1.575 GHz, an infrared dot projector at 940 nm for face recognition, and a camera for light of about 550 nm.
(i) Find the wavelength of the GPS signal and name its region. (1 mark)
(ii) Find the frequency of the dot projector and name its region. (1 mark)
(iii) Find the frequency of the light seen by the camera. (1 mark)
(iv) Arrange the three in increasing order of frequency. (1 mark)

Try it first, then show the solutionSolution

(i) $λ = \frac{3 \times 1 0 ^{8}}{1.575 \times 1 0 ^{9}} \approx 0.19$ m: radio waves.
(ii) $f = \frac{3 \times 1 0 ^{8}}{940 \times 1 0 ^{- 9}} \approx 3.19 \times 1 0^{14}$ Hz: infrared (longer than 700 nm).
(iii) $f = \frac{3 \times 1 0 ^{8}}{550 \times 1 0 ^{- 9}} \approx 5.45 \times 1 0^{14}$ Hz: visible.
(iv) GPS < infrared < visible.

Answer: (i) ≈ 0.19 m, radio; (ii) ≈ $3.19 \times 1 0^{14}$ Hz, IR; (iii) ≈ $5.45 \times 1 0^{14}$ Hz; (iv) GPS, IR, visible.

Where students lose marks in The Electromagnetic Spectrum

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

Mistake: Confusing microwaves with radio waves, UV or IR
The ISC 2024 and 2025 examiners reported both.
Wrong working
$1 0^{11}$ Hz called "radio waves".
Right working
$λ = \frac{3 \times 1 0 ^{8}}{1 0 ^{11}} = 3$ mm: microwaves.
Do this: Calculate λ and compare with the ranges.
Usually costs: 1 mark.
Mistake: Not converting nm to m
In 2025 candidates used λ = 6 instead of $6 \times 1 0^{- 9}$ m and lost the frequency mark.
Do this: Convert on the first line; 1 nm = $1 0^{- 9}$ m.
Usually costs: 1 mark.
Mistake: Speed of sound for radio waves
In 2024 some candidates gave 330 m/s or omitted the unit.
Do this: All electromagnetic waves: $3 \times 1 0^{8}$ m/s in vacuum.
Usually costs: 1 mark.
Mistake: Wrong unit for frequency
Frequency written in m or m/s.
Do this: Frequency is in hertz (Hz).
Usually costs: ½ to 1 mark.

Exam tips

Learn one source, one detector and one use for each region.
Write the order once at the start of an answer and use it.
For naming questions, show the λ calculation; it earns marks even if the name slips.
Accept that boundaries overlap; quote approximate ranges.

Practice questions on The 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
Night-vision cameras detect:
(a) ultraviolet
(b) infrared
(c) X-rays
(d) microwaves
Hint
Warm bodies radiate it.
Answer
(b).
Q2 · 2 marks · Short answerBoard level
Find the frequency of radiation of wavelength 250 nm and name the region.
Hint
Between 1 nm and 400 nm.
Answer
$1.2 \times 1 0^{15}$ Hz; ultraviolet.
Q3 · 1 mark · Short answerHarder board level
Arrange in decreasing order of frequency: radio waves, gamma rays, visible light, microwaves.
Hint
Gamma is highest.
Answer
Gamma rays, visible light, microwaves, radio waves.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Gamma rays and radio waves travel at the same speed in vacuum.
Reason (R): The speed $\frac{1}{μ _{0} ε _{0}}$ does not depend on wavelength.
(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.
Hint
Constants of free space.
Answer
(a).
Q5 · 2 marks · Short answerHarder board level
A radiation has frequency $3 \times 1 0^{21}$ Hz. Find its wavelength in nm, name the region and give one use.
Hint
$λ = 1 0^{- 13}$ m.
Answer
$1 0^{- 4}$ nm; gamma rays; cancer treatment or sterilising equipment.
Q6 · 2 marks · Short answerHarder board level
Name the radiation in each case: (a) absorbed by the ozone layer; (b) used to see a fractured bone; (c) used by a TV remote; (d) used to send signals to a satellite.
Hint
One region each.
Answer
(a) ultraviolet; (b) X-rays; (c) infrared; (d) microwaves.
Q7 · 2 marks · Short answerHOTS / competency
A microwave oven heats a cup of water quickly, but the same power of radio waves passes through it with little heating. Suggest why, using the idea of absorption by molecules.
Hint
Water molecules respond strongly at microwave frequencies.
Answer
Water molecules absorb energy strongly at microwave frequencies (GHz), so their motion and the temperature rise; at radio frequencies the absorption by water is very small, so most of the energy passes through.
Q8 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a satellite carries a thermal camera that records radiation of wavelength 10 µm from the Earth and a radio link at 3 GHz.
(i) Find the frequency of the thermal radiation and name the region. (1 mark)
(ii) Find the wavelength of the radio link. (1 mark)
(iii) Which one could detect warm and cool areas at night? Why? (1 mark)
(iv) Do the two signals reach the Earth at the same speed? (1 mark)
Hint
Warm bodies emit IR.
Answer
(i) $3 \times 1 0^{13}$ Hz; infrared. (ii) 0.1 m (on the radio-microwave boundary). (iii) The thermal (infrared) camera: all warm bodies emit infrared, even in the dark. (iv) Yes, both at $3 \times 1 0^{8}$ m/s in vacuum.

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.Order of increasing frequency?Show answer
Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
2.Visible wavelength range?Show answer
About 400 nm to 700 nm.
3.Speed of X-rays in vacuum?Show answer
$3 \times 1 0^{8}$ m/s.
4.λ = 3 m: frequency?Show answer
100 MHz.
5.Source of gamma rays?Show answer
Radioactive nuclei.

The Electromagnetic Spectrum: frequently asked questions

What is the difference between radio waves and microwaves?

Only wavelength. Radio waves are longer than about 0.1 m; microwaves run from 0.1 m to 1 mm, are made by magnetrons or klystrons, and are used in radar, satellite links and ovens.

Do all electromagnetic waves travel at the same speed?

Yes in vacuum: 3 \times 10^8 m/s. In a medium the speed depends on the medium and the wavelength.

Why are UV, X-rays and gamma rays harmful?

They have high frequency, so each photon carries enough energy to ionise atoms and damage living cells.

What does the ISC 2027 syllabus need on the spectrum?

A qualitative account of all regions from radio to gamma: production, detection, other properties and uses, and the approximate ranges of λ or f, or at least their correct order.

How has the spectrum been asked recently?

ISC 2024 Q5 (2 marks) asked the radiation of 10^{11} Hz and the speed of radio waves; ISC 2025 Q7 (2 marks) asked the wave from a satellite to a dish and the frequency for λ = 6 nm.