The electromagnetic spectrumIB MYP Physics: Revision notes
Section 1
The seven types of electromagnetic wave
The electromagnetic (EM) spectrum is a family of waves with the same basic nature. In order of increasing frequency (and decreasing wavelength):
radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma
Visible light is only a very small part of the spectrum. Its colours run from red (lowest frequency, longest wavelength) to violet (highest frequency, shortest wavelength).
Section 3
Wave speed equation: v = fλ
For any wave: speed = frequency × wavelength, or v = fλ.
In a vacuum all EM waves have the same speed, so if the frequency goes up the wavelength must go down.
Worked example. A microwave has a frequency of 2.5 GHz (2.5 × 10⁹ Hz). Find its wavelength.
λ = v / f = 3.0 × 10⁸ / 2.5 × 10⁹ = 0.12 m.
Remember: 1 MHz = 10⁶ Hz and 1 GHz = 10⁹ Hz.
Convert MHz and GHz to Hz before you use v = fλ. Forgetting this gives answers that are 10⁶ or 10⁹ times wrong.
Section 4
Uses of EM waves
- Radio waves: radio and TV broadcasting, mobile phone and Bluetooth communication.
- Microwaves: cooking food, Wi-Fi, satellite communication.
- Infrared: remote controls, heaters, toasters, thermal imaging cameras.
- Visible light: seeing, photography, optical fibres.
- Ultraviolet: sterilising water, security markings, sun lamps.
- X-rays: imaging bones and teeth, airport security scanners.
- Gamma rays: sterilising medical equipment and killing cancer cells.
Section 5
Dangers of EM waves
The danger grows with frequency, because high-frequency waves carry more energy.
- Microwaves can heat body cells (internal heating).
- Infrared can cause skin burns.
- Ultraviolet can damage skin cells, causing sunburn and skin cancer, and can damage the eyes.
- X-rays and gamma rays are ionising: they can knock electrons out of atoms and damage the DNA in cells. This can kill cells or cause cancer.
People reduce the risk with sunscreen, sunglasses, lead shielding and short exposure times.
When asked about the dangers of X-rays and gamma rays, use the word 'ionising' and link it to damage to cells or DNA and then cancer.
Must know
- Order of increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- All are transverse and travel at 3 × 10⁸ m/s in a vacuum.
- v = fλ; higher frequency means shorter wavelength.
- Uses depend on the wave; X-rays, gamma and UV are the most dangerous.
- X-rays and gamma rays are ionising and can cause cancer.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The electromagnetic spectrum
- A smartphone uses several parts of the electromagnetic spectrum. It uses radio waves for mobile calls, microwaves for Wi-Fi, infrared for face recognition and visible light from its screen.Radio waves have a longer wavelength than visible light. State how the frequency of radio waves compares with that of visible light and explain why.2 marks
- A local radio station broadcasts at a frequency of 100 MHz (1.0 × 10⁸ Hz). A home Wi-Fi router uses microwaves with a frequency of 2.4 GHz (2.4 × 10⁹ Hz). All electromagnetic waves travel at 3.0 × 10⁸ m/s in air.Calculate the wavelength of the microwaves used by the Wi-Fi router.2 marks
- A student tests how well different sunscreens block ultraviolet (UV) radiation. She uses special beads that change from white to purple when UV hits them, and scores the colour from 0 (white) to 5 (darkest purple). Each sample of beads is covered with clear plastic film. She spreads a thin layer of sunscreen on the film, then shines the same UV lamp on the sample for 10 minutes. The film with no sunscreen gives a score of 5. The film with SPF 15 sunscreen gives 3, SPF 30 gives 2 and SPF 50 gives 1.State a hypothesis for this investigation, with a scientific reason, and identify one control variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).