Electromagnetic Waves (VCE SSCE Physics): Revision Notes

Electromagnetic Waves

Introduction to electromagnetic waves

Light is a form of electromagnetic radiation that plays a vital role in our lives. Through photosynthesis, light sustains life on Earth. We use electromagnetic radiation in medicine, communications, energy production, weather forecasting, scientific research, and in understanding the Universe. For example, the James Webb Space Telescope captures infrared light from distant regions of space, allowing us to observe events from billions of years ago.

What is light?

For centuries, scientists debated the nature of light. Isaac Newton believed light consisted of tiny particles, while others thought it was a type of wave. In 1862, Scottish physicist James Clerk Maxwell proposed a revolutionary idea: light is made of waves consisting of vibrating electric and magnetic fields that can travel through a vacuum at $3.00 \times 10^8 \, \text{m} \cdot \text{s}^{-1}$ without needing a medium.

This self-propagating wave can travel through empty space, unlike mechanical waves (such as sound) which require a medium to travel through. This was a revolutionary concept because it meant electromagnetic waves could propagate through the vacuum of space.

Structure of electromagnetic waves

Electric field: An electric field is a region of space that exerts a force on charged particles. Electric fields are produced by charged particles and by changing magnetic fields. A changing electric field also produces a changing magnetic field.

Magnetic field: A magnetic field is a region of space that exerts a force on moving charged particles and magnetic materials. Magnetic fields are produced by changing electric fields and by magnetic materials. A changing magnetic field also produces a changing electric field.

Electromagnetic wave: An electromagnetic wave is a transverse wave consisting of perpendicular oscillating electric and magnetic fields that can propagate through space without requiring a medium.

The key features of electromagnetic waves are:

• The electric field ($E$) and magnetic field ($B$) oscillate perpendicular to each other
• Both fields are perpendicular to the direction the wave travels
• All electromagnetic waves are transverse waves
• They travel at the speed of light, $c = 3.00 \times 10^8 \, \text{m} \cdot \text{s}^{-1}$, in a vacuum
• They can travel through empty space without a medium

The wave equation for electromagnetic waves

All electromagnetic waves obey the wave equation. In a vacuum, all electromagnetic waves travel at the same speed, equal to $c$, the speed of light. This gives us the fundamental equation:

$c = f\lambda$

Where:

• $c$ = speed of light in vacuum ($3.00 \times 10^8 \, \text{m} \cdot \text{s}^{-1}$)
• $f$ = frequency of the wave (Hz)
• $\lambda$ = wavelength of the wave (m)

In media other than a vacuum, electromagnetic waves travel more slowly. For example, green light travels at $2.25 \times 10^8 \, \text{m} \cdot \text{s}^{-1}$ in water.

The electromagnetic spectrum

Electromagnetic spectrum: The electromagnetic spectrum is the complete range of all types of electromagnetic waves, arranged by their wavelengths and frequencies.

Maxwell correctly predicted that a whole family of electromagnetic waves could exist, with different wavelengths and frequencies. This range is called the electromagnetic spectrum. The spectrum is divided into approximate regions based on wavelength, because this helps us identify the different processes that create electromagnetic waves and classify their properties and applications.

The diagram shows that frequency increases from right to left as wavelength decreases from left to right. This inverse relationship is explained by the equation $c = f\lambda$. Since $c$ is constant in a vacuum, increasing frequency must result in decreasing wavelength.

The electromagnetic spectrum regions, in order of increasing frequency (and therefore decreasing wavelength) are:

1. Radio waves - longest wavelength, lowest frequency
2. Microwaves
3. Infrared (IR)
4. Visible light
5. Ultraviolet (UV)
6. X-rays
7. Gamma rays - shortest wavelength, highest frequency

Regions of the electromagnetic spectrum

Radio waves

Radio waves are the region of the electromagnetic spectrum with the longest wavelengths and lowest frequencies.

• Wavelength: Greater than $1 \, \text{m}$
• Frequency: Less than $300 \, \text{MHz}$

Microwaves

Microwaves are the region of the electromagnetic spectrum with wavelengths between radio waves and infrared radiation.

• Wavelength: $1 \, \text{m}$ to $1 \, \text{mm}$
• Frequency: $300 \, \text{MHz}$ to $300 \, \text{GHz}$

Infrared (IR)

Infrared is the region of the electromagnetic spectrum between microwaves and visible red light. Infrared radiation is produced by sources of heat and cannot be seen by human eyes, but can be detected as warmth on the skin.

• Wavelength: $1 \, \text{mm}$ to $700 \, \text{nm}$
• Frequency: $300 \, \text{GHz}$ to $430 \, \text{THz}$

Visible light

Visible light is the only region of the electromagnetic spectrum that can be detected by human eyes. It spans from red light (longest wavelength in visible range) to violet light (shortest wavelength in visible range).

• Wavelength: $700 \, \text{nm}$ to $400 \, \text{nm}$
• Frequency: $4.3 \times 10^{14} \, \text{Hz}$ to $7.5 \times 10^{14} \, \text{Hz}$

Ultraviolet (UV)

Ultraviolet is the region of the electromagnetic spectrum between visible light and X-rays.

• Wavelength: $400 \, \text{nm}$ to $10 \, \text{nm}$
• Frequency: $7.5 \times 10^{14} \, \text{Hz}$ to $3 \times 10^{16} \, \text{Hz}$

X-rays

X-rays are the region of the electromagnetic spectrum between UV and gamma rays. They have very short wavelengths and very high frequencies.

• Wavelength: $10 \, \text{nm}$ to $10 \, \text{pm}$
• Frequency: $3 \times 10^{16} \, \text{Hz}$ to $3 \times 10^{19} \, \text{Hz}$

Gamma rays

Gamma rays are the region of the electromagnetic spectrum with the shortest wavelengths and highest frequencies.

• Wavelength: Less than $10 \, \text{pm}$
• Frequency: Greater than $3 \times 10^{19} \, \text{Hz}$

Applications of the electromagnetic spectrum in society

Each region of the electromagnetic spectrum has important and diverse applications in society. The properties of each region determine how we can use them.

Radio waves and microwaves

Radio wave applications:

• Communication technologies: AM and FM radio, television, CB radios, mobile phones
• Radioastronomy: observing the Universe
• Navigation: aircraft and ship navigation (radio direction finding)
• Global Positioning Systems (GPS)
• Metal detectors
• Magnetic Resonance Imaging (MRI)
• Radio Frequency Identification (RFID) chips

Microwave applications:

• Heating in microwave ovens
• Radar systems
• Remote sensing: microwaves penetrate clouds, haze, dust and rain better than visible light
• Microwave astronomy
• Communications including mobile phones

Infrared, visible light, ultraviolet and X-rays

Infrared applications:

• Thermal imaging, including night vision goggles
• Remote controls for TVs and other devices
• Heat lamps in infrared saunas
• Infrared astronomy
• Optical fibre communication signals

Visible light applications:

• Vision - allows us to see our environment
• Lighting
• Optical astronomy
• Laser applications: surgery, laser pointers, cutting tools, communication
• Phototherapy: treatment for seasonal affective disorder (SAD) and jaundice
• Photosynthesis: enables plant growth

Ultraviolet applications:

• Vitamin D generation in human skin
• UV astronomy
• Medical and food sterilisation
• Detection of counterfeit money

X-ray applications:

• X-ray astronomy
• Industrial scanning for quality control
• Medical imaging including Computerised Tomography (CT) scans
• Cancer treatment (radiotherapy)
• Security imaging at airports and other locations
• Detection of art forgeries

Gamma rays

Gamma ray applications:

• Medical and food sterilisation
• Medical imaging using PET (Positron Emission Tomography) scanners
• Cancer treatment (radiotherapy)
• Gamma ray astronomy
• Industrial fault finding and tracing
• Electrostatic discharging

Why different applications for different regions?

Each region of the spectrum is also useful for different types of astronomy, allowing us to detect different objects and processes occurring in space.

Electromagnetic radiation from the Sun

Electromagnetic radiation: This is another term commonly used to refer to electromagnetic waves.

Almost all energy that Earth receives from the Sun arrives as electromagnetic radiation. Understanding the distribution of this radiation is crucial for making best use of solar energy and avoiding its dangers.

The graph shows how solar radiation is distributed across different wavelengths. The composition of sunlight reaching Earth is:

• Visible light: $43\%$ (wavelength $0.4$ to $0.7 \, \mu\text{m}$)
• Near infrared: $38\%$
• Ultraviolet: $7\%$
• Far infrared: $11\%$
• Other wavelengths (including microwaves and radio waves): Less than $1\%$

We can detect visible light with our eyes, infrared with our skin (as warmth), and ultraviolet with instruments (or through skin damage if exposure is excessive).

Benefits of solar radiation

• Solar cells: absorb visible and infrared light to generate electricity
• Phototherapy: visible light can treat seasonal affective disorder (SAD)
• Vitamin D production: UV radiation in our skin produces vitamin D
• Photosynthesis: plants use light for growth, with the most effective wavelengths being blue light (around $435 \, \text{nm}$) and red light (around $650 \, \text{nm}$)

Dangers of solar radiation

Working with units in the electromagnetic spectrum

Because the electromagnetic spectrum covers such a wide range of values, we use SI prefixes to make numbers more manageable. You must be able to convert between different units.

SI prefixes

Name	Value (times base unit)	Example
tera (T)	$10^{12}$	THz (terahertz)
giga (G)	$10^9$	GHz (gigahertz)
mega (M)	$10^6$	MHz (megahertz)
kilo (k)	$10^3$	km (kilometre)
(Base unit)	$10^0 = 1$	metre, hertz, gram
centi (c)	$10^{-2}$	cm (centimetre)
milli (m)	$10^{-3}$	mm (millimetre)
micro (μ)	$10^{-6}$	μm (micrometre)
nano (n)	$10^{-9}$	nm (nanometre)
pico (p)	$10^{-12}$	pm (picometre)

Converting units

To convert from a prefixed unit to the base unit: Multiply by the prefix value.

To convert from the base unit to a prefixed unit: Divide by the prefix value.

Remember!