Cambridge IGCSE Physics 0625 – Topic 3.3 Electromagnetic Spectrum
Learning Objective
Know that **all electromagnetic (EM) waves travel at the same high speed in a vacuum** (the speed of light, c).
Key Concepts
- EM waves are transverse: the electric field (**E**) and magnetic field (**B**) oscillate at right angles to each other and to the direction of travel.
- Universal speed in vacuum: In the absence of material media every EM wave propagates at the constant speed
c = 3.00 × 10⁸ m s⁻¹.
- Speed in air: The speed of light in dry air differs from c by less than 1 % (≈ 2.9 × 10⁶ m s⁻¹), so for IGCSE calculations it can be treated as c.
- Fundamental constant:
c = 1 ⁄ √(ε₀μ₀)
(ε₀ = 8.85 × 10⁻¹² F m⁻¹, μ₀ = 4π × 10⁻⁷ H m⁻¹).
Deriving this relationship is beyond the IGCSE syllabus; it is shown only to illustrate why the speed is a universal constant.
- Relation between speed, wavelength and frequency:
c = λ f (λ = wavelength, f = frequency).
This equation lets you convert between λ and f for any region of the spectrum.
- All EM waves share the same speed: The values of λ and f vary from radio waves to gamma rays, but c remains unchanged.
- Photon‑energy reminder (optional): For a deeper understanding, the energy of a photon is
E = h f (h = 6.63 × 10⁻³⁴ J s).
This is useful when deciding whether radiation is ionising (E > ≈ 3 eV). The syllabus does not require the formula, but many exam questions expect the concept.
Order of the Electromagnetic Spectrum
From longest wavelength / lowest frequency to shortest wavelength / highest frequency:
- Radio waves
- Microwaves
- Infrared (IR)
- Visible light
- Ultraviolet (UV)
- X‑rays
- Gamma rays
Health & Safety – Effects of Excessive Exposure
- Radio & microwaves: non‑ionising; can cause heating of tissue (dielectric heating). Example – microwave ovens, radar.
- Infrared: non‑ionising; intense IR can produce thermal burns to skin and eyes.
- Visible light: generally safe, but very bright sources can damage the retina.
- Ultraviolet: non‑ionising but high‑energy; can damage DNA → sunburn, premature ageing, increased skin‑cancer risk.
- X‑rays & Gamma rays: ionising; can eject tightly bound electrons, leading to cell damage, radiation sickness, and cancer. Shielding with lead or concrete is essential.
Electromagnetic Spectrum – Reference Table
| Region |
Wavelength λ (m) |
Frequency f (Hz) |
Typical Uses / Sources |
| Radio |
10³ – 10⁻¹ |
10⁵ – 10⁹ |
Broadcasting, TV, radar, mobile‑phone communication |
| Microwave |
10⁻¹ – 10⁻³ |
10⁹ – 10¹² |
Cooking (microwave ovens), satellite links, radar |
| Infrared (IR) |
10⁻³ – 7 × 10⁻⁷ |
10¹² – 4 × 10¹⁴ |
Thermal imaging, remote‑control signals, heating lamps |
| Visible |
7 × 10⁻⁷ – 4 × 10⁻⁷ |
4 × 10¹⁴ – 7.5 × 10¹⁴ |
Human vision, illumination, photography |
| Ultraviolet (UV) |
4 × 10⁻⁷ – 10⁻⁹ |
7.5 × 10¹⁴ – 3 × 10¹⁶ |
Sunlight, sterilisation, fluorescence, black lights |
| X‑ray |
10⁻⁹ – 10⁻¹¹ |
3 × 10¹⁶ – 3 × 10¹⁹ |
Medical imaging, security scanners, crystallography |
| Gamma |
< 10⁻¹¹ |
> 3 × 10¹⁹ |
Radioactive decay, nuclear reactions, astrophysical phenomena |
Sample Calculation – Frequency of Green Light
Find the frequency of green light with wavelength λ = 5.00 × 10⁻⁷ m (500 nm).
- Write the speed‑wavelength‑frequency relation: c = λ f.
- Rearrange for frequency: f = c ⁄ λ.
- Insert the known values:
f = (3.00 × 10⁸ m s⁻¹) ⁄ (5.00 × 10⁻⁷ m)
- Calculate:
f = 6.0 × 10¹⁴ Hz
- Interpretation: Green light oscillates 6 × 10¹⁴ times each second while travelling at the universal speed c.
Practice Questions
- Radio wavelength: A radio station broadcasts at a frequency of 100 MHz. What is the wavelength of this radio wave in a vacuum? (Use c = 3.00 × 10⁸ m s⁻¹.)
- UV frequency & ionising nature: Ultraviolet light has a wavelength of 2.0 × 10⁻⁸ m. Calculate its frequency and state whether the radiation is ionising. (Recall that ionising radiation has photon energy > ≈ 3 eV.)
Answers
- λ = c ⁄ f = (3.00 × 10⁸ m s⁻¹) ⁄ (1.00 × 10⁸ s⁻¹) = 3.0 m.
- f = c ⁄ λ = (3.00 × 10⁸ m s⁻¹) ⁄ (2.0 × 10⁻⁸ m) = 1.5 × 10¹⁶ Hz**.
Photon energy (optional): E = hf = (6.63 × 10⁻³⁴ J s)(1.5 × 10¹⁶ Hz) ≈ 1.0 × 10⁻¹⁷ J ≈ 6.2 eV.
Since 6.2 eV > 3 eV, the UV radiation is **ionising**.
Summary
- All electromagnetic waves travel at the same speed c = 3.00 × 10⁸ m s⁻¹ in a vacuum, regardless of wavelength or frequency.
- The spectrum runs from long‑wavelength, low‑frequency radio waves to short‑wavelength, high‑frequency gamma rays.
- Differences between regions lie in λ and f, not in speed; the relation c = λ f lets you convert between them.
- In air the speed is effectively c (difference < 1 %).
- Awareness of health effects—thermal (radio, microwave, IR), non‑ionising (UV), and ionising (X‑ray, gamma)—is essential for safe use of EM radiation.
- For deeper insight, remember the photon‑energy formula E = hf; it helps identify ionising radiation.