1. What is the electromagnetic spectrum?
The electromagnetic spectrum is the full range of all possible electromagnetic waves arranged according to their wavelength or frequency. Although the waves in the spectrum look very different—from long radio waves to tiny gamma rays—they are all the same type of wave: oscillating electric and magnetic fields.
The only difference between one region of the spectrum and another is the value of frequency \(f\) or wavelength \(\lambda\).
1.1. How EM waves differ across the spectrum
All electromagnetic waves travel at the same speed in vacuum (\(c = 3 \times 10^8\,\text{m/s}\)). But their frequencies and wavelengths vary widely:
- Long wavelength → low frequency → low energy
- Short wavelength → high frequency → high energy
1.2. Basic wave relation
Every part of the spectrum follows:
\(c = f \lambda\)
2. Order of the electromagnetic spectrum
The spectrum can be arranged in order of increasing frequency (and decreasing wavelength). A common memory trick is to go from biggest wavelength to smallest.
2.1. From lowest to highest frequency
- Radio waves
- Microwaves
- Infrared
- Visible light
- Ultraviolet
- X-rays
- Gamma rays
2.2. Visible light region
Only a tiny portion of the electromagnetic spectrum can be detected by the human eye. This is the visible region:
- Violet (highest frequency in visible)
- Indigo
- Blue
- Green
- Yellow
- Orange
- Red (lowest frequency in visible)
3. Radio waves
Radio waves have the longest wavelengths in the spectrum. They are used mainly for communication because they can travel long distances and bend around obstacles.
3.1. Wavelength and frequency range
- Wavelength: several meters to many kilometers
- Frequency: a few Hz to around \(10^9\,\text{Hz}\)
3.2. Common uses
- FM/AM radio broadcasting
- Television signals
- Mobile communication
- Wi-Fi and Bluetooth
4. Microwaves
Microwaves have shorter wavelengths than radio waves and interact differently with matter. They can heat food, detect objects, and transmit signals.
4.1. Wavelength and frequency range
- Wavelength: around 1 meter to 1 millimeter
- Frequency: \(10^9\) to \(10^{12}\,\text{Hz}\)
4.2. Common uses
- Microwave ovens
- Radar technology
- Satellite communication
- GPS systems
5. Infrared waves
Infrared waves are mainly experienced as heat. Anything warm emits infrared radiation.
5.1. Wavelength and frequency range
- Wavelength: about 1 millimeter to 700 nanometers
- Frequency: \(10^{12}\) to \(4 \times 10^{14}\,\text{Hz}\)
5.2. Common uses
- TV remote controls
- Thermal imaging cameras
- Night-vision equipment
- Heat lamps
6. Visible light
This is the narrow band of wavelengths that the human eye can detect. Different wavelengths correspond to different colours.
6.1. Wavelength and frequency range
- Wavelength: 700 nm (red) to 400 nm (violet)
- Frequency: \(4 \times 10^{14}\) to \(7.5 \times 10^{14}\,\text{Hz}\)
6.2. Examples in daily life
- Sunlight and daylight
- Light from bulbs and LEDs
- Lasers
- Rainbow colours
7. Ultraviolet radiation
Ultraviolet (UV) has more energy than visible light. The Sun emits UV radiation, and excessive exposure can be harmful.
7.1. Wavelength and frequency range
- Wavelength: 400 nm to 10 nm
- Frequency: \(7.5 \times 10^{14}\) to \(3 \times 10^{16}\,\text{Hz}\)
7.2. Common uses
- Sterilisation and disinfection
- Fluorescent lamps
- Detecting forgeries
- Medical phototherapy
8. X-rays
X-rays have even shorter wavelengths and higher frequencies. They carry enough energy to pass through soft tissues but not through bone.
8.1. Wavelength and frequency range
- Wavelength: 10 nm to 0.01 nm
- Frequency: \(3 \times 10^{16}\) to \(3 \times 10^{19}\,\text{Hz}\)
8.2. Common uses
- Medical imaging
- Security scanners
- Material analysis
9. Gamma rays
Gamma rays have the highest frequencies and shortest wavelengths in the spectrum. They carry enormous energy and can penetrate most materials.
9.1. Wavelength and frequency range
- Wavelength: less than 0.01 nm
- Frequency: greater than \(10^{19}\,\text{Hz}\)
9.2. Common sources and uses
- Nuclear reactions
- Radioactive decay
- Cancer treatment (radiotherapy)
- Astrophysical events (supernovae, pulsars)
10. Energy of electromagnetic waves across the spectrum
The energy carried by an electromagnetic wave photon depends on its frequency:
\(E = h f\)
Higher frequency → higher energy. This is why gamma rays are extremely energetic while radio waves carry very little energy per photon.
10.1. Meaning of the relation
The constant \(h\) is Planck’s constant. This formula connects classical wave ideas with quantum physics, and is essential for understanding light–matter interaction.