Electromagnetic Spectrum
Section: Waves | Syllabus: Cambridge IGCSE Physics (0625)
Properties of Electromagnetic Waves
- Electromagnetic (EM) waves are transverse waves - oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel
- All electromagnetic waves travel at the same high speed in a vacuum: c = 3.0 × 10^8 m/s, and approximately the same speed in air
- EM waves do not need a medium - they can travel through the vacuum of space
- Like all waves, they obey the wave equation c = fλ
An electromagnetic wave: perpendicular electric and magnetic field oscillations
The Electromagnetic Spectrum
The electromagnetic spectrum is the complete range of electromagnetic waves. It is divided into seven main regions:
The electromagnetic spectrum
- In order of increasing frequency (and equivalently, decreasing wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays
- Radio waves have the lowest frequency and longest wavelength; gamma rays have the highest frequency and shortest wavelength
| Region | Approximate Wavelength | Approximate Frequency |
|---|---|---|
| Radio waves | > 0.1 m | < 3 × 10^9 Hz |
| Microwaves | 1 mm - 0.1 m | 3 × 10^9 - 3 × 10^11 Hz |
| Infrared | 700 nm - 1 mm | 3 × 10^11 - 4 × 10^14 Hz |
| Visible light | 400 - 700 nm | 4 × 10^14 - 7.5 × 10^14 Hz |
| Ultraviolet | 10 - 400 nm | 7.5 × 10^14 - 3 × 10^16 Hz |
| X-rays | 0.01 - 10 nm | 3 × 10^16 - 3 × 10^19 Hz |
| Gamma rays | < 0.01 nm | > 3 × 10^19 Hz |
Uses of Electromagnetic Waves
Radio Waves
- Radio and television transmissions
- Astronomy
- Radio frequency identification (RFID)
Microwaves
- Satellite television
- Mobile phones (cell phones)
- Microwave ovens
Infrared
- Electric grills
- Short-range communications, such as remote controllers for televisions
- Intruder alarms
- Thermal imaging
- Optical fibres
Visible Light
- Vision
- Photography
- Illumination
Ultraviolet
- Security marking
- Detecting fake bank notes
- Sterilising water
X-rays
- Medical scanning
- Security scanners
Gamma Rays
- Sterilising food and medical equipment
- Detection of cancer and its treatment
Harmful Effects of Electromagnetic Radiation
Excessive exposure to some regions of the electromagnetic spectrum is harmful:
- Microwaves: internal heating of body cells
- Infrared: skin burns
- Ultraviolet: damage to surface cells and eyes, leading to skin cancer and eye conditions
- X-rays and gamma rays: mutation or damage to cells in the body
Communication Systems Using Electromagnetic Waves
Satellite Communication
- Communication with artificial satellites is mainly by microwaves
- Some satellite phones use low orbit artificial satellites
- Some satellite phones and direct broadcast satellite television use geostationary satellites
Low orbit and geostationary satellites
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Why These Waves Are Used
- Mobile phones and wireless internet use microwaves, because microwaves can penetrate some walls and only require a short aerial for transmission and reception
- Bluetooth uses radio waves, because radio waves pass through walls, although the signal is weakened by doing so
- Optical fibres (carrying visible light or infrared) are used for cable television and high-speed broadband, because glass is transparent to visible light and some infrared, and visible light and short-wavelength infrared can carry high rates of data
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Digital and Analogue Signals
- An analogue signal varies continuously - it can take any value within a range
- A digital signal takes only discrete values - typically just two, referred to as 1 (on) and 0 (off)
- A sound can be transmitted as either a digital or an analogue signal
Museli records her voice with a microphone, which produces a continuously varying (analogue) electrical signal. This signal can be sent directly as an analogue signal, or it can be sampled at regular intervals and converted into binary numbers to be sent as a digital signal.
Benefits of Digital Signalling
- Digital transmission allows an increased rate of transmission of data, compared with analogue
- Digital transmission allows an increased range, because of accurate signal regeneration: as a digital signal travels, noise is added and the pulses become distorted, but a repeater only needs to decide whether each pulse is closer to a 1 or a 0, and can then generate a fresh, clean pulse - removing the noise completely
- An analogue signal cannot be regenerated in this way: any noise added to it becomes a permanent part of the signal, and is amplified along with it at each stage
Analogue signal degradation versus digital signal regeneration
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