Dispersion and the Electromagnetic Spectrum

Q: What is the correct order of colours in the visible spectrum?

The order from longest to shortest wavelength is Red, Orange, Yellow, Green, Blue, Indigo, Violet, remembered using the mnemonic ROYGBIV.

CIE IGCSE Physics· 3.2.4, 3.3· 25 min read

🎯 Learning objectives

1.
Define dispersion of white light and recall the order of colours in the visible spectrum
2.
State the shared properties of all electromagnetic waves
3.
Recall the order, key uses and dangers of each component of the EM spectrum
4.
(Extended only) Explain dispersion via differing refractive indices for different wavelengths
5.
(Extended only) Define monochromatic light as visible light of a single frequency
6.
(Extended only) Distinguish analogue and digital signals, know that sound can be transmitted either way, and state the benefits of digital signalling (higher transmission rate and greater range via signal regeneration)
7.
(Extended only) Describe EM communication systems: microwaves (mobile phones, wifi), radio waves (Bluetooth) and optical fibres (visible light, infrared)
8.
Apply EM spectrum properties to solve structured exam questions

1. Dispersion of White Light (Core)★★☆☆☆⏱ 6 min

📘 Definition

Dispersion

The splitting of white light into its component colours when passing through a transparent medium such as a glass prism.

Example:

White light shone through a triangular prism produces a rainbow-like spectrum of 7 colours.

White light is a mixture of all visible colours, each with a unique wavelength. When entering a prism, each colour bends (refracts) by a different amount: red (longest visible wavelength) refracts the least, while violet (shortest visible wavelength) refracts the most, creating a clear visible spectrum.

📐 Worked Example

A student shines a narrow beam of white light at a triangular glass prism. State the colour of light that appears closest to the base of the prism, and the colour closest to the apex of the prism.

1
Step 1: Recall that shorter wavelength light refracts more when passing through glass.
2
Step 2: Violet light has the shortest visible wavelength, so it bends the most towards the base of the prism.
3
Step 3: Red light has the longest visible wavelength, so it bends the least, appearing closest to the apex of the prism.

Exam tip:

Always list the visible spectrum from longest to shortest wavelength unless instructed otherwise, as this is the standard exam convention.

2. Common Properties of Electromagnetic Waves (Core)★★☆☆☆⏱ 5 min

📘 Definition

Electromagnetic Spectrum

The continuous range of electromagnetic waves arranged in order of decreasing wavelength (or increasing frequency), from radio waves (longest wavelength, lowest frequency) to gamma rays (shortest wavelength, highest frequency).

All EM waves are transverse waves
All EM waves travel at $3 \times 1 0^{8}$ m/s in a vacuum
All EM waves transfer energy from a source to an absorber
All EM waves can be reflected, refracted, and diffracted

c = f \lambda

Where $c$ = speed of EM wave in m/s, $f$ = frequency in Hz, and $λ$ = wavelength in m.

📐 Worked Example

A local radio station broadcasts EM waves with a frequency of 95 MHz. Calculate the wavelength of these waves, assuming they travel at $3 \times 1 0^{8}$ m/s.

1
Step 1: Convert frequency to Hz: 95 MHz = $95 \times 1 0^{6}$ Hz = $9.5 \times 1 0^{7}$ Hz
2
Step 2: Rearrange $c = f λ$ to solve for wavelength:
3
$\lambda = \frac{c}{f}$
4
Step 3: Substitute values and calculate:
5
$\lambda = \frac{3 \times 10^8}{9.5 \times 10^7} \approx 3.2 \text{ m}$

Exam tip:

Remember MHz = 10⁶ Hz and GHz = 10⁹ Hz when solving wave speed calculations, otherwise you will get a wrong order of magnitude.

3. Components of the EM Spectrum (Core)★★★☆☆⏱ 8 min

The EM spectrum is ordered from longest to shortest wavelength as: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. You must recall 1-2 key uses and dangers for each wave type for the exam.

Wave Type	Key Uses	Dangers
Radio waves	Radio/TV broadcasting, long-distance communication	No known significant dangers at low intensities
Microwaves	Cooking food, satellite/mobile phone communication	High doses heat internal body tissue
Infrared	Heating systems, remote controls, thermal imaging	High doses cause skin burns
Visible light	Sight, photography, fibre-optic communication	Very bright light damages the eye retina
Ultraviolet	Sterilising equipment, sun tanning, fluorescent lighting	Causes skin cancer, eye damage, and premature ageing
X-rays	Medical bone imaging, airport security, weld inspection	Damages cells, causes mutations and cancer
Gamma rays	Sterilising medical tools, cancer radiotherapy, leak detection	Damages cells, causes mutations and cancer

📐 Worked Example

State which type of EM wave is used for each application: (a) Taking images of broken bones in hospital, (b) Sending signals to satellites orbiting Earth.

1
Step 1: X-rays penetrate soft body tissue but are absorbed by dense material like bone, so they are used for medical bone imaging.
2
Step 2: Microwaves pass through the Earth's atmosphere without being absorbed, so they are used for satellite communication.

4. Extended Supplement: Dispersion and EM Wave Applications★★★★☆Extended only⏱ 6 min

For Extended tier, you are required to explain why dispersion occurs, and connect uses of EM waves directly to their physical properties, rather than just recalling uses.

Dispersion occurs because glass has a slightly different refractive index for each wavelength of light. Longer wavelength (red) light has a lower refractive index in glass, so it refracts less. Shorter wavelength (violet) light has a higher refractive index, so it refracts more. This difference in refraction splits white light into its component colours.

📘 Definition

Monochromatic light

Visible light of a single frequency (and therefore a single wavelength and a single colour). This contrasts with white light, which is a mixture of many frequencies. A laser is a common source of monochromatic light.

📐 Worked Example

Explain why ultraviolet radiation is suitable for sterilising surgical equipment.

1
Step 1: Recall the relevant property of ultraviolet radiation: it has a high frequency, so it carries enough energy to destroy the genetic material of bacteria, viruses and other pathogens.
2
Step 2: Link the property to the use: the high energy of UV waves kills all microorganisms on surgical equipment, making the equipment safe for use on patients.

Exam tip:

When asked to explain a use of an EM wave in Extended papers, always explicitly link the use to a specific property of the wave to gain full marks.

5. Extended: Analogue and Digital Signals, and EM Communication Systems★★★★☆Extended only⏱ 8 min

📘 Definition

Analogue and digital signals

An analogue signal varies continuously and can take any value between its limits. A digital signal has only two possible values (on/off), usually written as the digits 1 and 0.

Sound can be transmitted as either an analogue signal or a digital signal. When sound is sent digitally it is first converted into a series of 1s and 0s before transmission, and converted back to sound at the receiver.

Higher transmission rate: digital signals can carry data at a faster rate than analogue signals.
Greater range with better quality: a digital signal can be regenerated (the original 1s and 0s are cleanly restored and unwanted noise is removed), so it can be sent over longer distances without losing quality. Noise picked up by an analogue signal cannot be removed in the same way.

Different regions of the electromagnetic spectrum are used for different communication systems:

EM region	Communication use
Microwaves	Mobile (cell) phone and wireless internet (wifi) communication, and satellite links
Radio waves	Bluetooth connections between devices, and radio/TV broadcasting
Visible light and infrared	Signals sent along optical fibres for high-speed broadband and cable communication

📐 Worked Example

For a telephone call: (a) describe the difference between an analogue and a digital signal, and (b) give one benefit of sending the call as a digital signal rather than an analogue signal.

1
Step 1: (a) An analogue signal varies continuously and can take any value, whereas a digital signal has only two values (1 and 0).
2
Step 2: (b) A digital signal can be regenerated to remove noise, so the call keeps its quality over a greater range; digital transmission also allows a higher rate of data transmission.

Exam tip:

For a digital-vs-analogue question, remember the two benefits of digital signalling: a higher rate of data transmission, and a greater range because the signal can be regenerated to remove noise.

6. Common Pitfalls

Wrong move:

Mixing up the order of the EM spectrum, listing gamma rays as the longest wavelength

Why:

Students often confuse frequency and wavelength order (frequency increases as wavelength decreases)

Correct move:

Use the Rabbits Mate In Very Unusual eXpensive Gardens mnemonic to remember longest wavelength first

Wrong move:

Stating EM waves travel at $3 \times 1 0^{8}$ m/s in glass or water

Why:

The speed of $3 \times 1 0^{8}$ m/s only applies in a vacuum; EM waves slow down in denser media

Correct move:

Only specify $3 \times 1 0^{8}$ m/s for vacuum, or state they travel at approximately this speed in air

Wrong move:

Stating red light refracts more than violet light during dispersion

Why:

Students incorrectly assume longer wavelengths bend more in glass

Correct move:

Remember shorter wavelength = higher refraction in glass, so violet bends most, red bends least

Wrong move:

Using frequency values in MHz or GHz directly in $c = f λ$ calculations without unit conversion

Why:

Unit conversion errors are common in wave calculations, leading to order of magnitude mistakes

Correct move:

Always convert frequency to Hz (1 MHz = 10⁶ Hz, 1 GHz = 10⁹ Hz) before substituting into the formula

Wrong move:

Mixing up uses of infrared and microwaves (e.g. stating microwaves are used for TV remote controls)

Why:

Both wave types are used for communication, so students confuse their use cases

Correct move:

Remember remote controls use infrared (short range, line of sight), while satellite/mobile communication uses microwaves (long range, pass through atmosphere)

7. Quick Reference Cheatsheet

Concept	Core Requirement	Extended Requirement
Dispersion	Recall order of visible spectrum (ROYGBIV), identify dispersion through a prism	Explain dispersion occurs due to differing refractive indices for different wavelengths
EM Wave Properties	Recall 4 shared EM wave properties, use $c = f λ$ for calculations	Same as Core, plus link properties to uses/dangers
EM Spectrum Order	Recall order from longest (radio) to shortest (gamma) wavelength	Same as Core, plus explain uses using wave properties
Uses & Dangers	Recall 1-2 key uses and dangers per wave type	Same as Core, plus explicitly link property to use/danger for exam marks
Monochromatic light		Visible light of a single frequency (a single colour/wavelength)
Analogue vs digital signals		Analogue = continuously varying; digital = only 1s and 0s; sound can be sent either way; digital benefits = higher rate + regeneration for greater range
EM communication systems		Microwaves: mobile phones & wifi; radio waves: Bluetooth; optical fibres: visible light & infrared

8. Frequently Asked

What is the correct order of colours in the visible spectrum?

The order from longest to shortest wavelength is Red, Orange, Yellow, Green, Blue, Indigo, Violet, remembered using the mnemonic ROYGBIV.

Do all EM waves travel at the same speed in a vacuum?

Yes! All electromagnetic waves travel at exactly 3×10⁸ m/s (the speed of light) in a vacuum, regardless of their wavelength or frequency.

What EM wave content do Core vs Extended learners need?

Core learners recall properties, order, uses and dangers. Extended learners also explain dispersion and link wave properties directly to their uses/dangers for full marks.

Going deeper

syllabusCIE IGCSE Physics 0625 2026-2028 SyllabusRefer to sections 3.2.4 and 3.3 for official requirements

What's Next

Now that you have mastered dispersion and the electromagnetic spectrum, you are ready to progress to the remaining wave topics in CIE IGCSE Physics 0625. Next, you will study sound waves, which are longitudinal mechanical waves that require a medium to travel, and apply the same $c = f λ$ formula you used for EM waves to sound calculations. You will also explore light reflection and refraction in more detail, including constructing ray diagrams for plane mirrors and lenses, which is a frequent structured question topic in both Core and Extended papers. Make sure to regularly practice recalling the EM spectrum order and its uses, as this is a high-frequency exam topic that earns easy marks if memorised correctly.