# Dispersion and the Electromagnetic Spectrum

> CIE IGCSE Physics · 0625 2026-2028
> Source: https://www.owlsprep.com/study/cie-0625-u3-dispersion-and-the-electromagnetic-spectrum/

This guide teaches you dispersion of white light, shared properties of electromagnetic waves, and the order, uses and dangers of the EM spectrum, aligned to CIE IGCSE Physics 0625 Core and Extended syllabuses.

**Prerequisites:** [Basic properties of transverse waves and the $c=f\lambda$ formula](https://www.owlsprep.com/study/cie-0625-u3-general-wave-properties/)

## Learning objectives

- Define dispersion of white light and recall the order of colours in the visible spectrum
- State the shared properties of all electromagnetic waves
- Recall the order, key uses and dangers of each component of the EM spectrum
- (Extended only) Explain dispersion via differing refractive indices for different wavelengths
- (Extended only) Define monochromatic light as visible light of a single frequency
- (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)
- (Extended only) Describe EM communication systems: microwaves (mobile phones, wifi), radio waves (Bluetooth) and optical fibres (visible light, infrared)
- Apply EM spectrum properties to solve structured exam questions

## Dispersion of White Light (Core)

**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.

> **mnemonic**
>
> ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet (order of visible spectrum from longest to shortest wavelength).

**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.

## Common Properties of Electromagnetic Waves (Core)

**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 $3 \times 10^8$ 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 $\lambda$ = 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 $3 \times 10^8$ m/s.

1. Step 1: Convert frequency to Hz: 95 MHz = $95 \times 10^6$ Hz = $9.5 \times 10^7$ Hz
2. Step 2: Rearrange $c = f\lambda$ 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.

## Components of the EM Spectrum (Core)

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.

> **mnemonic**
>
> Rabbits Mate In Very Unusual eXpensive Gardens: Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma (order of EM spectrum from longest to shortest wavelength).

| 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.

## Extended Supplement: Dispersion and EM Wave Applications

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.

**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.

## Extended: Analogue and Digital Signals, and EM Communication Systems

**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.

## Common pitfalls

- **Wrong:** Mixing up the order of the EM spectrum, listing gamma rays as the longest wavelength
  - Why it fails: Students often confuse frequency and wavelength order (frequency increases as wavelength decreases)
  - Correct: Use the *Rabbits Mate In Very Unusual eXpensive Gardens* mnemonic to remember longest wavelength first
- **Wrong:** Stating EM waves travel at $3 \times 10^8$ m/s in glass or water
  - Why it fails: The speed of $3 \times 10^8$ m/s only applies in a vacuum; EM waves slow down in denser media
  - Correct: Only specify $3 \times 10^8$ m/s for vacuum, or state they travel at approximately this speed in air
- **Wrong:** Stating red light refracts more than violet light during dispersion
  - Why it fails: Students incorrectly assume longer wavelengths bend more in glass
  - Correct: Remember shorter wavelength = higher refraction in glass, so violet bends most, red bends least
- **Wrong:** Using frequency values in MHz or GHz directly in $c=f\lambda$ calculations without unit conversion
  - Why it fails: Unit conversion errors are common in wave calculations, leading to order of magnitude mistakes
  - Correct: Always convert frequency to Hz (1 MHz = 10⁶ Hz, 1 GHz = 10⁹ Hz) before substituting into the formula
- **Wrong:** Mixing up uses of infrared and microwaves (e.g. stating microwaves are used for TV remote controls)
  - Why it fails: Both wave types are used for communication, so students confuse their use cases
  - Correct: Remember remote controls use infrared (short range, line of sight), while satellite/mobile communication uses microwaves (long range, pass through atmosphere)

## 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 $c=f\lambda$ 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 |

## 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 $c=f\lambda$ 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.

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-0625-u3-dispersion-and-the-electromagnetic-spectrum/