Electromagnetic Spectrum

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Notes

Electromagnetic Waves

The **electromagnetic spectrum** is a continuous range of waves arranged by **wavelength** or **frequency**.
Longer wavelength → lower frequency; shorter wavelength → higher frequency.
Order from longest to shortest wavelength: **radio waves**, **microwaves**, **infrared**, **visible light** (ROYGBIV), **ultraviolet**, **X-rays**, **gamma rays**.
All EM waves are **transverse** waves that transfer energy.
All EM waves travel at the same speed in a vacuum: **3.0 × 10⁸ m/s** (also approximately the speed in air).
Visible light is only a small part of the EM spectrum.

Uses of Electromagnetic Waves

**Radio waves**: radio/TV transmissions, astronomy, RFID.
**Microwaves**: satellite TV, mobile phones, microwave ovens, WiFi.
**Infrared**: remote controls, thermal imaging, optical fibres, intruder alarms.
**Visible light**: vision, photography, illumination.
**Ultraviolet**: security marking, detecting fake bank notes, sterilising water, fluorescence.
**X-rays**: medical scanning, security scanners (absorbed by bone, pass through tissue).
**Gamma rays**: sterilising food/equipment, cancer detection and treatment.

Dangers of Electromagnetic Waves

Higher frequency EM waves carry more energy and can **ionise** atoms (UV, X-rays, gamma rays).
**Microwaves**: internal heating of body cells (water absorption).
**Infrared**: skin burns.
**Ultraviolet**: skin cancer, eye damage; protection via sunscreen and sunglasses.
**X-rays and gamma rays**: cell mutation, DNA damage, cancer; radiographers use lead screens.
Ionising radiation risk increases with frequency and dose.

Communications

**Microwaves** are used for satellite communication (mobile phones, satellite TV) because they are not absorbed/reflected by the atmosphere.
**Radio waves** diffract around hills (similar wavelength to obstacles), used for local radio and Bluetooth.
**Optical fibres** use visible light or infrared for high-speed broadband; high frequency allows high data rates.
**Geostationary satellites**: orbit at 36 000 km, period 24 h, used for broadcasting; longer time delay.
**Low‑orbit/polar satellites**: orbit at ~200 km, shorter delay, used for weather and imaging.

Digital & Analogue Signals (Extended)

**Analogue signals** vary continuously and can take any value.
**Digital signals** have only two states (0/1, high/low).
Digital signals can be **regenerated** with minimal noise, giving larger range and higher data rate.
Extra data can be added to digital signals for error checking.
Digital sampling converts an analogue signal to digital by recording at set time intervals.

The electromagnetic spectrum showing the seven regions in order of decreasing wavelength (increasing frequency).

Practice questions

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1.Which of the following lists the electromagnetic spectrum in order of decreasing wavelength?
Easy
- ARadio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays
- BGamma rays, X-rays, ultraviolet, visible, infrared, microwaves, radio waves
- CRadio waves, infrared, microwaves, visible, ultraviolet, gamma rays, X-rays
- DVisible, infrared, radio waves, microwaves, ultraviolet, X-rays, gamma rays
2.All electromagnetic waves travel at the same speed in a vacuum. What is this speed?
Easy
- A3.0 × 10⁸ m/s
- B3.0 × 10⁶ m/s
- C3.0 × 10¹⁰ m/s
- D3.0 × 10³ m/s
3.Which type of electromagnetic radiation is used for television remote controls?
Easy
- AInfrared
- BRadio waves
- CUltraviolet
- DMicrowaves
4.Which of the following is a harmful effect of excessive exposure to ultraviolet radiation?
Easy
- ASkin cancer
- BInternal heating of body cells
- CSkin burns
- DMutation of DNA
5.Which type of electromagnetic radiation has a wavelength of about 1 km?
Medium
- ARadio waves
- BMicrowaves
- CInfrared
- DVisible light
6.A geostationary satellite orbits at a height of 36 000 km above the Earth's surface. What is its orbital period?
Medium
- A24 hours
- B12 hours
- C48 hours
- D72 hours
7.Which property increases as you move from radio waves to gamma rays in the electromagnetic spectrum?
Medium
- AFrequency
- BWavelength
- CSpeed in a vacuum
- DAmplitude
8.Which type of signal can only take one of two discrete values?
Medium
- ADigital signal
- BAnalogue signal
- CRadio signal
- DMicrowave signal

Slides

The Electromagnetic Spectrum

The electromagnetic spectrum is a continuous range of waves arranged by wavelength or frequency.
From longest to shortest wavelength: Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays, Gamma rays.
All EM waves are transverse and travel at the same speed in a vacuum (3.0 × 10⁸ m/s).

Which of the following is the correct order of the electromagnetic spectrum from longest to shortest wavelength?

Gamma, X-rays, Ultraviolet, Visible, Infrared, Microwaves, Radio
Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays, Gamma
Radio, Infrared, Visible, Ultraviolet, Microwaves, X-rays, Gamma
Visible, Infrared, Radio, Microwaves, Ultraviolet, X-rays, Gamma

Properties of EM Waves

All EM waves travel at the same speed in a vacuum: 3.0 × 10⁸ m/s.
They are transverse waves that transfer energy from source to absorber.
Wavelength and frequency are inversely related: longer wavelength means lower frequency.

All electromagnetic waves travel through a vacuum with the same ____.

Uses of Radio and Microwaves

Radio waves: radio and TV transmissions, astronomy, RFID.
Microwaves: satellite TV, mobile phones, microwave ovens, WiFi.
Many wireless communications use microwaves, not radio waves.

Which type of electromagnetic radiation is used for satellite television?

Radio waves
Microwaves
Infrared
Visible light

Uses of Infrared, Visible, and Ultraviolet

Infrared: thermal imaging, remote controls, optical fibres, intruder alarms.
Visible light: vision, photography, illumination.
Ultraviolet: security marking, detecting fake bank notes, sterilising water, fluorescence.

Which of these are uses of infrared radiation?

Television remote controllers
Sterilising medical equipment
Thermal imaging cameras
Detecting fake bank notes

Uses of X-rays and Gamma Rays

X-rays: medical scanning, security scanners (absorbed by dense materials like bone).
Gamma rays: sterilising food/medical equipment, cancer detection and treatment.
Both can ionise atoms and damage living tissue.

Which property of X-rays makes them useful for medical imaging?

They are absorbed by bone but pass through soft tissue
They have a very long wavelength
They are not ionising
They travel slower than visible light

Dangers of EM Waves

Higher frequency EM waves carry more energy and can ionise atoms.
Ultraviolet: skin cancer, eye damage.
X-rays and gamma rays: mutation or damage to cells, cancer.
Microwaves: internal heating of body cells at high intensities.

Excessive exposure to ____ can cause skin cancer and eye conditions.

Communications with Satellites

Geostationary satellites: orbit at 36,000 km, 24-hour period, used for TV and telecoms.
Low orbit (polar) satellites: orbit at ~200 km, used for weather, military, imaging.
Microwaves are used for satellite communications.

What is the orbital period of a geostationary satellite?

12 hours
24 hours
36 hours
48 hours

Radio Waves and Diffraction

Radio waves have long wavelengths (around a kilometre) and can diffract around hills.
This allows radio signals to be received even when the transmitter is not in line of sight.
TV signals (shorter wavelength) do not diffract as much, so they may be blocked by hills.

Which statements about radio waves are true?

Radio waves can diffract around hills.
Radio waves have shorter wavelengths than visible light.
Radio waves are used for Bluetooth.
Radio waves are ionising.

Digital and Analogue Signals

Analogue signals vary continuously and can take any value.
Digital signals have only two states: 0 or 1 (high/low).
Digital signals can be regenerated, have less noise, and allow error checking.

Which is an advantage of digital signals over analogue signals?

They can take any value
They are continuously varying
They can be regenerated with minimal noise
They have a shorter range

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