PhysicsICSE Class 10 · Spectrum and Scattering of Light

Electromagnetic Spectrum: Order, Common Properties, Infrared and Ultraviolet (ICSE Class 10)

Q: What is the order of the electromagnetic spectrum in increasing wavelength?

Gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves. In increasing frequency the order is reversed.

Q: What properties are common to all electromagnetic radiations?

They need no medium, travel at 3 × 10⁸ m/s in vacuum, are transverse, can be reflected and refracted, and are not deflected by electric or magnetic fields.

Q: Why is a quartz prism used for ultraviolet and a rock-salt prism for infrared?

Ordinary glass absorbs both ultraviolet and infrared. Quartz lets ultraviolet through and rock salt lets infrared through, so each is used to obtain and study that part of the spectrum.

Updated 15 October 2026 · Fact-checked

In order of increasing wavelength the electromagnetic spectrum is gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves. All of them are transverse waves that need no medium and travel at 3 × 10⁸ m/s in vacuum, so c = fλ: a longer wavelength always means a lower frequency.

Understand Electromagnetic Spectrum: Order, Properties, IR and UV

One family, many members. Visible light is only a thin slice of a much wider family of waves called electromagnetic radiations. Each member is the same kind of wave; they differ only in wavelength (and so frequency). Arranged in order, they form the electromagnetic spectrum.

The order (CISCE asks names only). In order of increasing wavelength, which is also decreasing frequency:

gamma rays → X-rays → ultraviolet → visible light → infrared → microwaves → radio waves

Read it backwards for increasing frequency. The visible part, from violet (about 400 nm) to red (about 700 nm), sits between ultraviolet and infrared: *ultra*violet is 'beyond violet' and *infra*red is 'below red'. The boundaries between regions are not sharp; neighbouring regions overlap.

Properties common to all electromagnetic radiations.

They need no material medium: they travel through vacuum (sunlight crosses empty space).
In vacuum they all travel at the same speed, $c = 3 \times 1 0^{8}$ m/s.
They are transverse waves.
They obey $c = f λ$ , so frequency and wavelength are inversely proportional.
They can be reflected and refracted and obey the same laws as light.
They carry no charge, so they are not deflected by electric or magnetic fields.
They carry energy from one place to another.

These are the properties examiners want when a question says 'name one property common to A and B'. A use (for example 'used in radar') is not a property, and 'long wavelength' is not common to all.

Speed, frequency and wavelength. Because $c$ is fixed in vacuum, $λ = \frac{c}{f}$ . Radio waves have frequencies of about $1 0^{6}$ to $1 0^{9}$ Hz and wavelengths of metres or more; X-rays have frequencies near $1 0^{18}$ Hz and wavelengths near $1 0^{- 10}$ m. Always change MHz, GHz, nm and cm to SI units first.

Infrared (IR): properties. Wavelength just longer than red (about 700 nm to 1 mm).

Given out by every hot body: the Sun, a heater, a hot iron, our own bodies.
Strong heating effect: it is absorbed by most materials and warms them, so it is often called heat radiation.
Detected by a thermometer with a blackened bulb or a thermopile, not by the eye.
Absorbed by ordinary glass, so its spectrum is studied with a rock-salt prism.
Scattered less than visible light, so it gets through haze and fog better.

Infrared: uses. TV and AC remote controls; night-vision and thermal-imaging cameras; physiotherapy (heat treatment of sore muscles); drying paint, fruit and vegetables; remote sensing and weather pictures from satellites; signalling.

Ultraviolet (UV): properties. Wavelength just shorter than violet (about 400 nm down to a few nm).

Produced by very hot bodies such as the Sun, by electric arcs (welding) and by mercury-vapour lamps.
Strong chemical effect: it darkens photographic film and starts chemical changes.
Causes fluorescence: some substances glow with visible light when UV falls on them.
Kills germs (bacteria and viruses).
Absorbed by ordinary glass, so its spectrum is studied with a quartz prism.
Mostly absorbed by the ozone layer; in excess it harms the skin and eyes.

Ultraviolet: uses. Sterilising drinking water, surgical instruments and food; detecting forged notes, documents and fake gems (fluorescence); helping the skin make vitamin D; producing light in fluorescent tubes.

Do not mix up ultraviolet (electromagnetic) with ultrasonic (sound above 20 kHz), or infrared with infrasonic (sound below 20 Hz). Sound is a mechanical wave and cannot cross vacuum. Also keep apart the gamma rays of this spectrum and the α and β particles of radioactivity: α and β are deflected by fields, electromagnetic waves are not.

Electromagnetic Spectrum: Order, Properties, IR and UV: formulas and key facts

Wave equation for every electromagnetic radiation: $c = f λ$; c = 3 × 10⁸ m/s in vacuum for all of them; f in Hz, λ in m.
Order of increasing wavelength: $γ < X < UV < visible < IR < microwave < radio$; The same list read backwards is the order of increasing frequency.
Unit conversions: $1 nm = 1 0^{- 9} m, 1 MHz = 1 0^{6} Hz, 1 GHz = 1 0^{9} Hz$; Convert before substituting in c = fλ.
One use of each radiation: Gamma rays killing cancer cells
X-rays detecting bone fractures
Ultraviolet sterilising water
Visible light seeing
Infrared remote controls
Microwaves radar and cooking
Radio waves radio and TV broadcasting; In order of increasing wavelength.
Infrared and ultraviolet compared: Position IR beyond red, UV beyond violet
Main effect IR heating, UV chemical and fluorescent
Detector IR blackened-bulb thermometer, UV photographic film
Prism IR rock salt, UV quartz
Typical source IR any hot body, UV Sun and mercury-vapour lamp; Both are absorbed by ordinary glass.

Gamma rays	killing cancer cells
X-rays	detecting bone fractures
Ultraviolet	sterilising water
Visible light	seeing
Infrared	remote controls
Microwaves	radar and cooking
Radio waves	radio and TV broadcasting

Position	IR beyond red, UV beyond violet
Main effect	IR heating, UV chemical and fluorescent
Detector	IR blackened-bulb thermometer, UV photographic film
Prism	IR rock salt, UV quartz
Typical source	IR any hot body, UV Sun and mercury-vapour lamp

How to solve Electromagnetic Spectrum: Order, Properties, IR and UV questions

Spectrum questions are either 'order and names', 'property or use', or a c = fλ calculation. Each type has a fixed routine.

1For an order question, write the full list gamma → X → UV → visible → IR → micro → radio first, then pick the requested items out of it in the same order (reverse it if the question asks for frequency).
2For a 'common property' question, choose from: no medium needed, same speed $3 \times 1 0^{8}$ m/s in vacuum, transverse, reflected and refracted, not deflected by fields. Never give a use.
3For a use or source question, match the radiation to its effect: IR heats, UV acts chemically and causes fluorescence, X-rays pass through flesh but not bone.
4For a calculation, convert to Hz and metres, then use $λ = \frac{c}{f}$ or $f = \frac{c}{λ}$ with $c = 3 \times 1 0^{8}$ m/s.
5Use the answer to name the region: about $1 0^{- 12}$ m gamma, $1 0^{- 10}$ m X-ray, $1 0^{- 8}$ m UV, $4$ to $7 \times 1 0^{- 7}$ m visible, $1 0^{- 5}$ m IR, cm to mm microwave, metres and more radio.

Electromagnetic Spectrum: Order, Properties, IR and UV 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.

Light from the Sun

Suppose you watch the Sun rise. It is about $1.5 \times 1 0^{11}$ m away, and space in between is almost a perfect vacuum.

How long ago did the light you see leave the Sun, and why can sound from the Sun never reach you?

Try it first, then show the solutionSolution

Light needs no medium: $t = \frac{d}{c} = \frac{1.5 \times 1 0 ^{11}}{3 \times 1 0 ^{8}} = 500$ s, a little over 8 minutes.
Sound is a mechanical wave and needs a material medium; it cannot cross vacuum.

Answer: About 500 s (over 8 minutes) ago; sound cannot travel through the vacuum of space.

Memory hook: Light hops across empty space; sound needs something to push.

Why you do not tan behind a window

Suppose you sit all afternoon in a sunny room behind closed glass windows. You feel warm but your skin does not tan.

Explain using the properties of UV and visible light.

Try it first, then show the solutionSolution

Tanning is a chemical change in the skin caused by ultraviolet.
Ordinary glass absorbs most UV, so little of it reaches you; visible light passes through the glass and warms whatever absorbs it, including you.

Answer: The glass absorbs the ultraviolet that causes tanning, while visible light passes through and warms you.

Memory hook: Glass stops UV: that is why a quartz prism is used to study it.

Tricks to remember Electromagnetic Spectrum: Order, Properties, IR and UV

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

The order in one sentence
'Good X-ray Units View Invisible Microscopic Rays': Gamma, X-ray, UV, Visible, Infrared, Microwave, Radio, in order of increasing wavelength.
Use it when: Any order question; read it backwards for increasing frequency.
Ultra and infra
Ultra means beyond (beyond violet, shorter λ); infra means below (below red, longer λ). Ultrasonic and infrasonic are sound, not light.
Use it when: Placing UV and IR and avoiding the ultrasonic trap.
Q for quartz, R for rock salt
Quartz is for UV (both have a 'short' feel: short wavelength); Rock salt is for the Red side (infrared).
Use it when: Prism-material questions.
Five common properties: 'No Speed Trap Reflects Fields'
No medium, same Speed in vacuum, Transverse, Reflected and refracted, not deflected by Fields.
Use it when: 'Name one property common to all' questions.

Electromagnetic Spectrum: Order, Properties, IR and UV: solved examples

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

Example 1: Longest wavelength · 1 mark · MCQBoard level

Which of the following has the longest wavelength?
(a) infrared
(b) X-rays
(c) microwaves
(d) ultraviolet

Try it first, then show the solutionSolution

Order of increasing wavelength: X-rays < ultraviolet < infrared < microwaves. Microwaves come last of the four.

Answer: (c) microwaves

Example 2: Completing the spectrum and naming a common property · 3 marks · Short answerHarder board level

In the list below the radiations are arranged in order of decreasing frequency, but two are missing:
Gamma rays, P, ultraviolet, visible, Q, microwaves, radio waves
(a) Name P and Q.
(b) Which of P and Q is used in night-vision cameras?
(c) State one property shared by P and radio waves.

Try it first, then show the solutionSolution

(a) Decreasing frequency is increasing wavelength, so P is X-rays and Q is infrared. (1)
(b) Q, infrared: warm bodies give out IR, which the camera detects. (1)
(c) Both travel through vacuum at $3 \times 1 0^{8}$ m/s (or: both need no medium; both are transverse; both can be reflected). (1)

Answer: (a) P: X-rays, Q: infrared (b) Infrared (c) Same speed $3 \times 1 0^{8}$ m/s in vacuum (any one common property)

Example 3: Wavelength of an FM station · 2 marks · Short answerBoard level

An FM radio station broadcasts at 96 MHz. Calculate the wavelength of its waves. ( $c = 3 \times 1 0^{8}$ m/s)

Try it first, then show the solutionSolution

$f = 96$ MHz $= 96 \times 1 0^{6}$ Hz. (½)
$λ = \frac{c}{f} = \frac{3 \times 1 0 ^{8}}{96 \times 1 0 ^{6}} = 3.125$ m. (1½)

Answer: $λ = 3.125$ m (radio waves)

Example 4: Microwave oven · 3 marks · Short answerHarder board level

A microwave oven uses radiation of frequency 2.45 GHz. (a) Find its wavelength in cm. (b) State one other use of microwaves. (c) Would these waves reach Earth from a satellite through empty space? Give a reason.

Try it first, then show the solutionSolution

(a) $λ = \frac{3 \times 1 0 ^{8}}{2.45 \times 1 0 ^{9}} \approx 0.122$ m $\approx 12.2$ cm. (1)
(b) Radar, or mobile-phone and satellite communication. (1)
(c) Yes: like all electromagnetic waves, microwaves need no material medium. (1)

Answer: (a) About 12.2 cm (b) Radar or communication (c) Yes, no medium is needed

Example 5: Frequency of an X-ray · 3 marks · Short answerHarder board level

X-rays used in a dental clinic have a wavelength of 0.2 nm. Calculate their frequency, and explain why the X-ray and the light from the clinic's lamp reach a patient's jaw at the same speed in air (take air as vacuum).

Try it first, then show the solutionSolution

$λ = 0.2 \times 1 0^{- 9}$ m $= 2 \times 1 0^{- 10}$ m. (½)
$f = \frac{c}{λ} = \frac{3 \times 1 0 ^{8}}{2 \times 1 0 ^{- 10}} = 1.5 \times 1 0^{18}$ Hz. (1½)
All electromagnetic radiations travel at the same speed $c$ in vacuum; only their frequency and wavelength differ. (1)

Answer: $1.5 \times 1 0^{18}$ Hz; all electromagnetic waves share the speed $3 \times 1 0^{8}$ m/s in vacuum

Example 6: Why the remote's beam is invisible · 3 marks · Short answerHOTS / competency

A TV remote control's LED emits radiation of wavelength 940 nm. (a) Calculate its frequency. (b) Compare it with the frequency of red light of wavelength 700 nm and hence explain why you cannot see the beam. (c) Name the region of the spectrum.

Try it first, then show the solutionSolution

(a) $f = \frac{3 \times 1 0 ^{8}}{940 \times 1 0 ^{- 9}} \approx 3.19 \times 1 0^{14}$ Hz. (1)
(b) Red light: $f = \frac{3 \times 1 0 ^{8}}{700 \times 1 0 ^{- 9}} \approx 4.29 \times 1 0^{14}$ Hz. The remote's frequency is lower (its wavelength longer) than the reddest light the eye can detect, so the eye does not respond to it. (1)
(c) Infrared, just beyond the red end. (1)

Answer: (a) About $3.19 \times 1 0^{14}$ Hz (b) Lower than red ( $4.29 \times 1 0^{14}$ Hz), outside the visible range (c) Infrared

Example 7: Case study: a radar pulse · 4 marks · Case-basedHOTS / competency

An airport radar sends out a short pulse of microwaves. It is reflected by an aircraft 30 km away and returns to the radar.
(a) Calculate the time between sending and receiving the pulse. ( $c = 3 \times 1 0^{8}$ m/s)
(b) Which property of electromagnetic waves allows the pulse to return?
(c) Would the time change if the radar used radio waves of longer wavelength? Explain.
(d) Why can a similar method with sound not be used to find the distance of a satellite in space?

Try it first, then show the solutionSolution

(a) Total path $= 2 \times 30 \times 1 0^{3} = 6 \times 1 0^{4}$ m, so $t = \frac{6 \times 1 0 ^{4}}{3 \times 1 0 ^{8}} = 2 \times 1 0^{- 4}$ s. (1)
(b) Electromagnetic waves can be reflected (they obey the laws of reflection). (1)
(c) No: all electromagnetic waves travel at the same speed in air (practically $c$ ), whatever their wavelength. (1)
(d) Sound needs a material medium and cannot travel through the vacuum of space. (1)

Answer: (a) $2 \times 1 0^{- 4}$ s (b) Reflection (c) No, same speed (d) Sound cannot cross vacuum

Where students lose marks in Electromagnetic Spectrum: Order, Properties, IR and UV

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

Mistake: Not knowing the order of the spectrum
Questions ask for the order by wavelength or by frequency, or for a missing member. Guessing one name usually breaks the whole order.
Wrong working
'Gamma, ultraviolet, X-rays, visible, microwaves, infrared, radio.'
Right working
'Gamma, X-rays, ultraviolet, visible, infrared, microwaves, radio' (increasing wavelength).
Do this: Learn one mnemonic and write the full list in the margin before answering.
Usually costs: CISCE examiners reported that candidates did not know the order by wavelength and frequency (2025 Q1(v)) and guessed the missing radiation (2024 Q4(ii)).
Mistake: Giving a use when a property is asked
A property describes the radiation itself (speed, medium, nature); a use describes what people do with it.
Wrong working
'One property common to microwaves and radio waves: they are used in communication' or 'they have long wavelength'.
Right working
'Both travel at $3 \times 1 0^{8}$ m/s in vacuum' or 'both need no medium'.
Do this: Learn the five common properties as a list and pick one.
Usually costs: Reported by CISCE examiners for 2024 Q4(ii).
Mistake: Confusing ultraviolet with ultrasonic, or infrared with infrasonic
The ultra/infra words sound alike, but ultrasonic and infrasonic are sound waves, which need a medium.
Wrong working
'Ultrasonic radiation is studied with a quartz prism.'
Right working
'Ultraviolet radiation is studied with a quartz prism.'
Do this: If the answer ends in -sonic it is sound; light-family answers end in -violet or -red.
Usually costs: Reported by CISCE examiners for 2025 Q5(iii).
Mistake: Mixing up electromagnetic radiations with radioactive radiations
Alpha and beta are charged particles deflected by fields; electromagnetic waves are not.
Wrong working
'All electromagnetic radiations are deflected by a magnetic field.'
Right working
'Electromagnetic radiations carry no charge, so they are not deflected by electric or magnetic fields.'
Do this: Only gamma rays belong to both lists, and they are not deflected.
Usually costs: Also reported for 2025 Q5(iii).
Mistake: Substituting MHz or nm directly
c is in m/s, so f must be in Hz and λ in metres.
Wrong working
$λ = \frac{3 \times 1 0 ^{8}}{96} = 3.1 \times 1 0^{6}$ m for a 96 MHz station.
Right working
$λ = \frac{3 \times 1 0 ^{8}}{96 \times 1 0 ^{6}} = 3.125$ m.
Do this: Write the conversion as a separate line before substituting.
Usually costs: The calculation mark.

Exam tips

When a question says 'increasing frequency', write the wavelength order and reverse it; do not try to build the frequency order from memory.
Name the prism material whenever a UV or IR spectrum is mentioned: quartz for ultraviolet, rock salt for infrared.
For remote sensing, infrared (also microwaves or radio waves) was accepted in 2025 Q5(iii); for traffic signals, visible light.
For 'property common to all', the safest answer is 'they travel at $3 \times 1 0^{8}$ m/s in vacuum without needing a medium'.
Give a calculated wavelength with its unit and, if asked, the region it falls in.

Practice questions on Electromagnetic Spectrum: Order, Properties, IR and UV

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
To obtain an infrared spectrum from sunlight, a physicist chooses a prism of:
(a) crown glass
(b) quartz
(c) rock salt
(d) flint glass
Hint
Glass absorbs infrared.
Answer
(c) rock salt
Q2 · 1 mark · Very short answerBoard level
Arrange in order of increasing frequency: microwaves, visible light, gamma rays, infrared, ultraviolet.
Hint
Increasing frequency is decreasing wavelength.
Answer
Microwaves, infrared, visible light, ultraviolet, gamma rays.
Q3 · 2 marks · Short answerHarder board level
A germicidal lamp gives out ultraviolet radiation of wavelength 250 nm. Calculate its frequency. ( $c = 3 \times 1 0^{8}$ m/s)
Hint
Change nm to m first.
Answer
$f = \frac{3 \times 1 0 ^{8}}{250 \times 1 0 ^{- 9}} = 1.2 \times 1 0^{15}$ Hz.
Q4 · 2 marks · Short answerHarder board level
A Wi-Fi router transmits at 5 GHz. (a) Find the wavelength in cm. (b) In which region of the spectrum does it lie?
Hint
$λ = \frac{c}{f}$ .
Answer
(a) $λ = \frac{3 \times 1 0 ^{8}}{5 \times 1 0 ^{9}} = 0.06$ m $= 6$ cm. (b) Microwaves.
Q5 · 2 marks · Short answerHarder board level
Radiation A has four times the frequency of radiation B. Both travel in vacuum. (a) What is the ratio of their speeds? (b) What is the ratio $λ_{A} : λ_{B}$ ?
Hint
Speed is the same for all; $λ \propto \frac{1}{f}$ .
Answer
(a) 1 : 1, since all electromagnetic waves travel at $c$ in vacuum. (b) $λ_{A} : λ_{B} = 1 : 4$ .
Q6 · 3 marks · Short answerBoard level
Give (a) two properties and (b) two uses of ultraviolet radiation. (c) Why is ultraviolet not the same as ultrasonic?
Hint
Properties describe what it does; uses describe what we do with it.
Answer
(a) Any two: causes fluorescence; strong chemical effect on photographic film; kills germs; absorbed by ordinary glass; mostly absorbed by the ozone layer. (b) Any two: sterilising water or instruments; detecting forged notes; helping the body make vitamin D. (c) Ultraviolet is an electromagnetic radiation beyond violet that can travel through vacuum; ultrasonic is sound of frequency above 20 kHz, a mechanical wave that needs a medium.
Q7 · 3 marks · Short answerHarder board level
Gamma rays from a radioactive source have a frequency of $3 \times 1 0^{20}$ Hz. Find their wavelength. Are they deflected when they pass between two charged plates? Give a reason.
Hint
Electromagnetic waves carry no charge.
Answer
$λ = \frac{3 \times 1 0 ^{8}}{3 \times 1 0 ^{20}} = 1 0^{- 12}$ m. No: gamma rays are electromagnetic waves and carry no charge, so electric (and magnetic) fields do not deflect them.
Q8 · 3 marks · Short answerHOTS / competency
A communication satellite is 36 000 km above a ground station. (a) How long does a signal take to travel up to the satellite? (b) The signal is a microwave; name one property of microwaves that makes this link possible. (c) A student says 'the signal would travel faster if infrared were used'. Comment.
Hint
Distance in metres, speed $c$ .
Answer
(a) $t = \frac{3.6 \times 1 0 ^{7}}{3 \times 1 0 ^{8}} = 0.12$ s. (b) It needs no material medium (it crosses the near-vacuum above the atmosphere) and travels at $3 \times 1 0^{8}$ m/s. (c) Wrong: all electromagnetic waves travel at the same speed in vacuum; infrared would take the same 0.12 s.
Q9 · 3 marks · Short answerHOTS / competency
A student fixes a sheet of ordinary glass in front of a thermopile placed beyond the red end of a spectrum from a rock-salt prism, and the reading falls almost to zero. (a) What radiation was the thermopile detecting? (b) Explain the fall. (c) What would happen to the reading if the rock-salt prism had been replaced by a glass prism from the start?
Hint
Which radiation does glass absorb?
Answer
(a) Infrared. (b) Ordinary glass absorbs infrared, so very little reaches the thermopile. (c) The reading beyond the red end would be very small from the start, because the glass prism itself would absorb most of the infrared; this is why rock salt is used.
Q10 · 3 marks · Short answerHarder board level
A hospital wants to (i) check whether a patient's wrist is broken, (ii) sterilise drinking water for a ward and (iii) ease a footballer's stiff thigh muscle with gentle heat. Name the radiation suited to each job and the property that makes it suitable. Which of the three has the shortest wavelength?
Hint
Match each job to an effect: passing through flesh, killing germs, heating.
Answer
(i) X-rays: they pass through flesh but are stopped by bone. (ii) Ultraviolet: it kills germs. (iii) Infrared: strong heating effect. Shortest wavelength: X-rays.

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.Radiation just beyond the violet end?Show answer
Ultraviolet.
2.Speed of all electromagnetic waves in vacuum?Show answer
$3 \times 1 0^{8}$ m/s.
3.Prism used for the infrared spectrum?Show answer
Rock salt.
4.Radiation with the highest frequency?Show answer
Gamma rays.
5.Radiation used to find bone fractures?Show answer
X-rays.
6.Effect of UV used to detect forged notes?Show answer
Fluorescence.

Electromagnetic Spectrum: Order, Properties, IR and UV: frequently asked questions

What is the order of the electromagnetic spectrum in increasing wavelength?

Gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves. In increasing frequency the order is reversed.

What properties are common to all electromagnetic radiations?

They need no medium, travel at 3 × 10⁸ m/s in vacuum, are transverse, can be reflected and refracted, and are not deflected by electric or magnetic fields.

Why is a quartz prism used for ultraviolet and a rock-salt prism for infrared?

Ordinary glass absorbs both ultraviolet and infrared. Quartz lets ultraviolet through and rock salt lets infrared through, so each is used to obtain and study that part of the spectrum.