Ch 6 · 3.3 Electromagnetic Spectrum · O Level Physics 5054
Topic 6 of 7 in Waves — O Level Physics 5054 — 49 lessons.
Introduction
Your phone, your microwave oven, the TV remote, your eyes, a sunbed, a hospital X-ray machine and a cancer-treatment unit all use the same kind of wave: electromagnetic radiation. They differ only in wavelength and frequency. Together they form the electromagnetic spectrum. (Syllabus section 3.3.)
Key ideas — The family of electromagnetic waves
All electromagnetic (EM) waves:
🧑🏫 Teacher's explanation
Imagine a massive orchestra where every musician is playing a different note. Some notes are very low and deep (long wavelengths, low frequency), while others are incredibly high and piercing (short wavelengths, high frequency). Even though the notes sound different, every single musician is playing at the exact same tempo.
📖 Definitions
🔍 Worked example — Easy
A radio station broadcasts at a frequency of 1.0 × 10^6 Hz. Calculate the wavelength of these waves in a vacuum.
🔍 Worked example — Standard
An X-ray has a wavelength of 2.0 × 10^-10 m. Calculate its frequency.
🔍 Worked example — Exam-style
A medical technician uses gamma rays to treat a tumor. If the gamma ray has a frequency of 3.0 × 10^20 Hz, calculate its wavelength in a vacuum.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the speed of the X-ray wave in a vacuum. [1] (b) Explain why X-rays are considered dangerous to humans. [2] [3 marks]
Key ideas — The regions in order
Memory aid: "Rabbits Meet In Very Unusual eXpensive Gardens."
🧑🏫 Teacher's explanation
Imagine the electromagnetic spectrum as a giant musical keyboard. On one end, you have the deep, low bass notes (radio waves) that travel very long distances and have very long wavelengths. On the other end, you have the extremely high-pitched, piercing notes (gamma rays) that have tiny wavelengths and carry massive amounts of energy.
📖 Definitions
🔍 Worked example — Easy
Arrange the following electromagnetic waves in order of increasing frequency: X-rays, Radio waves, Infrared, Ultraviolet.
🔍 Worked example — Standard
A radio wave has a frequency of 1.0 × 10^6 Hz. Calculate its wavelength in a vacuum, given the speed of light c = 3.0 × 10^8 m/s.
🔍 Worked example — Exam-style
An X-ray photon has a frequency of 3.0 × 10^18 Hz. Calculate its wavelength in a vacuum.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the speed of the X-ray waves in a vacuum. [1] (b) Explain why X-rays are considered more dangerous to human tissue than infrared radiation. [2] [3 marks]
Key ideas — Uses
🧑🏫 Teacher's explanation
Imagine the electromagnetic (EM) spectrum as a giant musical keyboard. The 'low notes' are radio waves—long, slow vibrations that can bend around corners like deep bass notes. The 'high notes' are gamma rays—extremely fast, high-energy vibrations that can punch through solid walls like a piercing whistle.
📖 Definitions
🔍 Worked example — Easy
A radio wave has a frequency of 1.0 × 10^6 Hz. Calculate its wavelength in a vacuum.
🔍 Worked example — Standard
An X-ray has a wavelength of 0.05 nm (5.0 × 10^-11 m). Calculate its frequency.
🔍 Worked example — Exam-style
A medical technician uses gamma rays to treat a tumor. If the gamma rays have a frequency of 3.0 × 10^20 Hz, what is their wavelength?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State why X-rays are suitable for this purpose. [1] (b) Explain why X-rays can be dangerous to the security officer if they are exposed to them excessively. [2] [3 marks]
Key ideas — Dangers
Heating. Too much exposure to microwaves and infrared can heat soft tissue (water in body tissue absorbs the energy) and cause burns.
🧑🏫 Teacher's explanation
Think of electromagnetic waves like a series of waves in the ocean. Low-frequency waves (like radio waves) are like gentle swells that barely move a sandcastle. However, as you move up the spectrum toward gamma rays, the waves become like massive, high-speed crashing breakers. These high-frequency waves don't just move things; they carry enough 'punch' to break things apart.
📖 Definitions
🔍 Worked example — Easy
A person is exposed to excessive infrared radiation from a heat lamp. Explain the primary biological effect on the person's skin.
🔍 Worked example — Standard
Compare the dangers of Ultraviolet (UV) radiation and Gamma rays in terms of their interaction with human cells.
🔍 Worked example — Exam-style
A radiographer is working in a room with an X-ray machine. State two safety measures they should take and explain why X-rays are dangerous.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why X-rays are considered dangerous to the technician. [2] (b) Suggest two ways the technician can protect themselves from the radiation. [2] [4 marks]
Worked example
Example 1. A mobile phone uses microwaves of frequency 1.8 × 10^9 Hz. Find the wavelength.
🔍 Worked example — Easy
A radio station broadcasts at a frequency of 95.0 MHz. Calculate the wavelength of these radio waves in a vacuum.
🔍 Worked example — Standard
A medical X-ray machine uses radiation with a wavelength of 0.05 nm. Calculate the frequency of the X-rays.
🔍 Worked example — Exam-style
An intruder alarm uses infrared radiation. If the speed of the wave is 3.0 × 10^8 m/s and the frequency is 3.0 × 10^13 Hz, calculate the wavelength.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, radio waves. (2) 3.0 × 10^8 m/s. (3) (a) infrared; (b) gamma rays; (c) ultraviolet; (d) microwaves. (4) (a) heating of soft tissue / burns; (b) skin cancer or cataracts. (5) f = 3.0 × 10^8/1500 = 2.0 × 10^5 Hz.