Light Waves
1. Nature of Light
Learning outcomes
- I can describe light as an electromagnetic wave.
- I can distinguish between light and mechanical waves.
- I can explain that light does not require a medium.
- I can identify visible light within the electromagnetic spectrum.
- I can describe the speed of light.
Nature of Light
Light is a form of electromagnetic radiation. It travels as an electromagnetic wave and transfers energy from one place to another.
Unlike mechanical waves such as sound, water waves, and seismic waves, light does not require particles of matter to carry its energy. Light can travel through empty space.
This is why light from the Sun and distant stars can cross the near-vacuum of space and reach Earth.
Visible light is only a small part of a much larger family of electromagnetic waves called the electromagnetic spectrum.
Light as an Electromagnetic Wave
An electromagnetic wave consists of changing:
- electric fields
- magnetic fields
These fields oscillate as the wave travels.
The electric field and magnetic field are perpendicular to each other, and both are perpendicular to the direction in which the wave travels.
Because the oscillations are perpendicular to the direction of travel, electromagnetic waves are classified as transverse waves.
Therefore:
light is an electromagnetic transverse wave.
Electric and Magnetic Fields
The word electromagnetic combines:
electric + magnetic
In an electromagnetic wave:
- the electric field changes
- the magnetic field changes
- the changing fields propagate through space
A simplified representation has:
electric field ⟂ magnetic field
and:
both fields ⟂ direction of wave travel
The symbol ⟂ means perpendicular to.
This is different from a mechanical transverse wave, where particles of matter physically oscillate.
Light Does Not Require a Medium
One of the most important properties of light is that it can travel through a vacuum.
A vacuum is a region containing extremely little matter.
Light does not need:
- air
- water
- glass
- particles
- another material medium
to continue travelling.
This is fundamentally different from mechanical waves.
Light from the Sun
The Sun is approximately 150 million kilometres from Earth.
Most of the space between the Sun and Earth contains extremely little matter.
If light required a material medium, sunlight could not travel across this space.
But electromagnetic waves can travel through a vacuum.
Therefore, energy from the Sun can reach Earth as electromagnetic radiation.
This energy is essential for:
- vision
- photosynthesis
- Earth's climate system
- solar energy technologies
Mechanical Waves
A mechanical wave requires a material medium through which to travel.
Examples include:
- sound waves
- water waves
- waves on strings
- seismic waves
The medium can be:
- solid
- liquid
- gas
Particles within the medium oscillate and transfer energy to neighbouring particles.
Light vs Mechanical Waves
| Property | Light | Mechanical Waves |
|---|---|---|
| Type | Electromagnetic | Mechanical |
| Requires a medium | No | Yes |
| Can travel through vacuum | Yes | No |
| Transfers energy | Yes | Yes |
| Has wavelength | Yes | Yes |
| Has frequency | Yes | Yes |
| Has wave speed | Yes | Yes |
| Can reflect | Yes | Yes |
| Can refract | Yes | Yes |
| Can diffract | Yes | Yes |
| Can interfere | Yes | Yes |
| What oscillates? | Electric and magnetic fields | Particles of a medium |
The major difference is therefore not whether they behave like waves.
Both do.
The key difference is:
mechanical waves require matter; electromagnetic waves do not.
Light and Sound
Light and sound provide a useful comparison.
Sound:
- mechanical wave
- requires a medium
- cannot travel through a vacuum
- sound in air is longitudinal
- much slower than light
Light:
- electromagnetic wave
- does not require a medium
- travels through a vacuum
- transverse
- travels extremely quickly
This explains why light and sound behave differently in many everyday situations.
Lightning and Thunder
During a thunderstorm, lightning and thunder are produced at approximately the same event.
However, you usually:
see the lightning first
and:
hear the thunder later
This happens because light travels enormously faster than sound.
The delay does not mean that the lightning occurred several seconds before the thunder was produced.
Instead, the two signals travel toward you at very different speeds.
The Electromagnetic Spectrum
Visible light is part of the electromagnetic spectrum.
The main regions, from lower frequency and longer wavelength toward higher frequency and shorter wavelength, are:
radio → microwave → infrared → visible → ultraviolet → X-ray → gamma ray
All of these are electromagnetic waves.
They differ mainly in their:
- frequency
- wavelength
- photon energy
Visible Light
Visible light is the portion of the electromagnetic spectrum that can normally be detected by the human eye.
It occupies only a very small part of the entire electromagnetic spectrum.
Visible wavelengths are approximately:
380–750 nm
The exact boundaries are approximate because human visual sensitivity does not suddenly stop at one precise wavelength.
A nanometre (nm) is:
1 nm = 0.000000001 m = 10⁻⁹ m
Colours of Visible Light
Visible light contains a range of colours.
A common sequence from longer wavelength toward shorter wavelength is:
red → orange → yellow → green → blue → violet
Red light has a longer wavelength than violet light.
Violet light has a higher frequency than red light.
Therefore, across the visible spectrum:
red → longer wavelength, lower frequency
violet → shorter wavelength, higher frequency
White Light
What we perceive as white light can contain many wavelengths of visible light.
A prism can separate white light into a spectrum of colours.
This happens because different wavelengths of visible light are refracted by slightly different amounts when passing through the prism.
The spreading of light into its component wavelengths is called:
dispersion
Rainbows
Rainbows provide a natural example of visible-light dispersion.
Sunlight enters water droplets in the atmosphere.
The light undergoes processes including:
- refraction
- internal reflection
- dispersion
Different visible wavelengths emerge at different angles.
This produces the familiar range of colours.
Wavelength of Light
Like other waves, light has a wavelength.
Symbol:
λ
Wavelength is the distance between corresponding points of successive wave cycles.
For visible light, wavelength is extremely small and is commonly measured in:
nanometres (nm)
For example, visible green light might have a wavelength near:
550 nm
Convert to metres:
550 nm = 550 × 10⁻⁹ m
= 5.50 × 10⁻⁷ m
Frequency of Light
Light also has a frequency.
Symbol:
f
Frequency is the number of complete oscillations occurring each second.
Unit:
hertz (Hz)
Visible-light frequencies are extremely high, on the order of hundreds of trillions of hertz.
Frequency and wavelength are inversely related when wave speed is fixed.
Therefore:
higher frequency → shorter wavelength
lower frequency → longer wavelength
The Speed of Light
The speed of light in a vacuum is one of the fundamental constants of physics.
Its exact defined value is:
c = 299 792 458 m/s
For most school calculations, this is rounded to:
c ≈ 3.00 × 10⁸ m/s
This means light travels approximately:
300 000 000 metres every second
or:
300 000 kilometres every second
The symbol c is normally used for the speed of light in a vacuum.
How Fast Is Light?
At approximately 300 000 km/s, light could theoretically travel a distance equal to about 7.5 times Earth's circumference in one second.
Even at this enormous speed, light still takes time to travel across astronomical distances.
For example, sunlight takes approximately:
8 minutes 20 seconds
to reach Earth.
When we observe the Sun, we therefore see it as it was several minutes earlier.
Light from the Moon
The Moon is roughly:
384 000 km
from Earth on average.
Light takes approximately:
time = distance/speed
time ≈ 384 000/300 000
time ≈ 1.28 s
to travel from the Moon to Earth.
This means the Moon we see is being observed as it was about 1.3 seconds earlier.
Light from Stars
Stars are much farther away.
Astronomers often describe these distances using light-years.
A light-year is a unit of distance, not time.
It is the distance light travels through a vacuum in one year.
One light-year is approximately:
9.46 × 10¹² km
When we observe a star 100 light-years away, we see light that began its journey approximately 100 years ago.
Astronomy therefore allows us to look into the past.
The Wave Equation for Light
The general wave relationship is:
v = fλ
For electromagnetic waves travelling in a vacuum:
c = fλ
where:
c = speed of light in vacuum
f = frequency
λ = wavelength
Because c is constant in a vacuum, frequency and wavelength must change inversely.
If frequency increases:
wavelength decreases
If wavelength increases:
frequency decreases
Worked Example: Finding Frequency
Suppose visible light has a wavelength of:
600 nm
First convert to metres:
600 nm = 600 × 10⁻⁹ m
λ = 6.00 × 10⁻⁷ m
Use:
c = fλ
Rearrange:
f = c/λ
Substitute:
f = (3.00 × 10⁸)/(6.00 × 10⁻⁷)
f = 5.00 × 10¹⁴ Hz
Worked Example: Finding Wavelength
An electromagnetic wave has a frequency of:
6.00 × 10¹⁴ Hz
Use:
c = fλ
Rearrange:
λ = c/f
Substitute:
λ = (3.00 × 10⁸)/(6.00 × 10¹⁴)
λ = 5.00 × 10⁻⁷ m
Convert to nanometres:
λ = 500 nm
This wavelength lies within the visible region.
Light Slows Down in Materials
The value:
3.00 × 10⁸ m/s
is the speed of light in a vacuum.
Light generally travels more slowly through materials such as:
- air
- water
- glass
- plastics
For example, light travels more slowly through glass than through a vacuum.
This change in speed is important in explaining refraction.
Light Entering a New Medium
When light crosses from one transparent medium into another:
- its speed can change
- its wavelength can change
- its direction can change
- its frequency remains the same across the boundary
For example, when light enters glass from air, it slows down.
Since:
v = fλ
and frequency remains constant, a decrease in speed means:
wavelength decreases
Refraction
Refraction is the change in direction of a wave associated with a change in wave speed as it enters a different medium.
Light commonly refracts when travelling between:
- air and water
- air and glass
- glass and air
This is why objects underwater can appear displaced from their actual positions.
It is also fundamental to the operation of lenses.
Reflection
Light can also undergo reflection.
Reflection occurs when light encounters a surface and some of the wave energy returns.
Examples include:
- mirrors
- reflections from water
- shiny metal surfaces
- reflected sunlight from the Moon
We see most ordinary objects because light reflects from them and enters our eyes.
Diffraction
Light can undergo diffraction, meaning that it can spread when passing through an opening or around an obstacle.
Diffraction is particularly noticeable when the size of the opening or obstacle is comparable to the wavelength.
Because visible-light wavelengths are extremely small, noticeable diffraction usually requires very small openings or structures.
Diffraction provides important evidence for the wave nature of light.
Interference
When light waves overlap, they can undergo interference.
The waves combine according to the principle of superposition.
They can produce:
- constructive interference
- destructive interference
Interference patterns provide further evidence that light behaves as a wave.
Polarization
Because light is a transverse electromagnetic wave, it can be polarized.
Polarization restricts the orientation of the electromagnetic oscillations.
Applications include:
- polarized sunglasses
- photography
- LCD displays
- scientific instruments
The ability of light to be polarized is strong evidence for its transverse nature.
Electromagnetic Waves Beyond Visible Light
Visible light represents only the portion our eyes detect.
The rest of the electromagnetic spectrum is invisible to human vision.
Radio waves
Used in communication and broadcasting.
Microwaves
Used in communication, radar, and microwave heating.
Infrared
Associated with thermal radiation and used in thermal imaging.
Visible light
Detected by human eyes.
Ultraviolet
Produced strongly by the Sun and used in several technologies.
X-rays
Used in imaging and scientific analysis.
Gamma rays
Very high-frequency electromagnetic radiation associated with nuclear and high-energy processes.
All are electromagnetic waves.
Comparing Regions of the Electromagnetic Spectrum
| Region | Relative Wavelength | Relative Frequency |
|---|---|---|
| Radio | Longest | Lowest |
| Microwave | Long | Low |
| Infrared | Shorter | Higher |
| Visible | Small range | High |
| Ultraviolet | Short | Higher |
| X-ray | Very short | Very high |
| Gamma | Shortest | Highest |
Moving from radio toward gamma:
wavelength decreases
while:
frequency increases
Visible Light's Position
Visible light lies between:
infrared
and:
ultraviolet
in the electromagnetic spectrum.
Therefore, the order around visible light is:
infrared → visible → ultraviolet
Infrared has:
- longer wavelength
- lower frequency
than visible light.
Ultraviolet has:
- shorter wavelength
- higher frequency
than visible light.
Light Carries Energy
Electromagnetic waves transfer energy.
Examples include:
- sunlight warming Earth's surface
- solar cells receiving energy from sunlight
- infrared radiation heating objects
- light producing chemical changes in photosynthesis
- electromagnetic radiation carrying information in communication systems
Light can therefore transfer energy even across empty space.
Light Carries Information
Light is also one of our most important sources of information.
When light reaches our eyes, it can tell us about:
- colour
- shape
- position
- movement
- brightness
- texture
Scientific instruments can analyze light to determine properties of distant objects.
Astronomers, for example, study electromagnetic radiation to investigate stars and galaxies that cannot be visited directly.
Light and Communication
Electromagnetic waves can carry information over large distances.
Visible and near-infrared light can be used in fibre-optic communication.
Information can be encoded into rapidly changing light signals and transmitted through optical fibres.
This allows large amounts of information to be transferred very quickly.
Light as More Than a Simple Classical Wave
Describing light as an electromagnetic wave explains phenomena such as:
- reflection
- refraction
- diffraction
- interference
- polarization
However, modern physics shows that light also has particle-like properties.
Light energy can be transferred in packets called:
photons
This is known as wave-particle duality.
For understanding the basic nature and propagation of light, the electromagnetic-wave model is extremely important.
Comparing Sound and Light in Space
Imagine an explosion occurring in space.
Visible light from the event could travel through the vacuum and potentially reach a distant observer.
Ordinary sound could not travel through the empty space because sound requires a material medium.
Therefore:
light can cross a vacuum
sound cannot cross a vacuum
This comparison clearly demonstrates the difference between electromagnetic and mechanical waves.
A Complete Example
A beam of visible light has:
λ = 450 nm
Convert to metres:
450 nm = 4.50 × 10⁻⁷ m
Use:
c = fλ
Rearrange:
f = c/λ
f = (3.00 × 10⁸)/(4.50 × 10⁻⁷)
f ≈ 6.67 × 10¹⁴ Hz
A wavelength of 450 nm lies in the visible region, toward the blue part of the spectrum.
The wave can travel through a vacuum because it is electromagnetic rather than mechanical.
A Reliable Strategy for Analyzing Light
When examining a problem involving light:
- Identify whether the wave is electromagnetic or mechanical.
- Determine whether a material medium is required.
- Identify the wavelength or frequency if provided.
- Determine where the radiation lies within the electromagnetic spectrum.
- Use c = 3.00 × 10⁸ m/s for light travelling in a vacuum.
- Use c = fλ when frequency or wavelength must be calculated in vacuum.
- Convert units carefully, especially nm to m.
- Remember that frequency and wavelength are inversely related for a fixed wave speed.
- If light enters another material, remember that its speed and wavelength can change while its frequency remains constant.
- Identify wave behaviours such as reflection, refraction, diffraction, interference, or polarization when relevant.
Common Mistakes
Thinking light requires air
Light can travel through a vacuum.
Thinking electromagnetic waves contain vibrating air particles
Electromagnetic waves consist of oscillating electric and magnetic fields.
Thinking visible light is the entire electromagnetic spectrum
Visible light is only a small region of the spectrum.
Thinking sound and light are the same type of wave
Sound is mechanical. Light is electromagnetic.
Thinking all electromagnetic waves are visible
Human eyes detect only the visible region.
Thinking red light has a higher frequency than violet light
Red has a longer wavelength and lower frequency than violet.
Thinking light always travels at exactly 3.00 × 10⁸ m/s
That value applies to a vacuum. Light travels more slowly through materials.
Thinking frequency changes when light enters glass
Frequency remains constant across the boundary; speed and wavelength change.
Thinking a light-year is a unit of time
A light-year is a unit of distance.
Forgetting to convert nanometres into metres
Remember:
1 nm = 10⁻⁹ m
before using SI units in calculations.
Key Terms
Light — electromagnetic radiation, with visible light being the portion detectable by human vision.
Electromagnetic wave — wave consisting of oscillating electric and magnetic fields.
Electromagnetic radiation — energy transmitted through space as electromagnetic waves.
Electric field — region in which an electric charge experiences an electric force.
Magnetic field — region in which magnetic effects can be detected.
Transverse wave — wave whose oscillations are perpendicular to the direction of travel.
Mechanical wave — wave that requires a material medium.
Medium — matter through which a mechanical wave travels.
Vacuum — region containing extremely little matter.
Electromagnetic spectrum — complete range of electromagnetic radiation arranged by wavelength or frequency.
Visible light — electromagnetic radiation detectable by the human eye.
Wavelength (λ) — distance between corresponding points on consecutive wave cycles.
Frequency (f) — number of complete oscillations per second.
Hertz (Hz) — SI unit of frequency.
Nanometre (nm) — one billionth of a metre, or 10⁻⁹ m.
Speed of light (c) — speed of electromagnetic radiation in a vacuum, approximately 3.00 × 10⁸ m/s.
Reflection — return of wave energy from a boundary.
Refraction — change in wave direction associated with a change in wave speed.
Diffraction — spreading of waves around obstacles or through openings.
Interference — combination of overlapping waves.
Polarization — restriction of transverse oscillations to particular orientations.
Dispersion — separation of light according to wavelength.
Photon — quantum, or packet, of electromagnetic energy.
Light-year — distance travelled by light through a vacuum in one year.
Key Takeaways
- Light is electromagnetic radiation.
- Light behaves as a transverse electromagnetic wave.
- Electromagnetic waves consist of oscillating electric and magnetic fields.
- The electric and magnetic fields are perpendicular to each other and to the direction of wave travel.
- Light does not require a material medium.
- Light can travel through a vacuum.
- Mechanical waves require matter through which to travel.
- Sound is mechanical, while light is electromagnetic.
- Visible light is only a small part of the electromagnetic spectrum.
- The electromagnetic spectrum includes radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma radiation.
- Visible light lies between infrared and ultraviolet.
- Red visible light has a longer wavelength and lower frequency than violet light.
- Violet light has a shorter wavelength and higher frequency than red light.
- Light travels through a vacuum at approximately 3.00 × 10⁸ m/s.
- The wave relationship for light in a vacuum is c = fλ.
- At constant speed, wavelength and frequency are inversely related.
- Light travels more slowly through materials than through a vacuum.
- When light enters another medium, its frequency remains constant while its speed and wavelength may change.
- Light can undergo reflection, refraction, diffraction, interference, and polarization.
- Light transfers energy and information.
- The ability of sunlight to cross space demonstrates that electromagnetic waves do not require a medium.
- Light from distant astronomical objects takes time to reach us.
- A light-year is a unit of distance.
- The electromagnetic-wave model explains many important properties of light, although modern physics also reveals particle-like behaviour through photons.
Check Your Understanding
- Describe light as an electromagnetic wave.
- What oscillates in an electromagnetic wave?
- Explain why light is classified as a transverse wave.
- Compare light and mechanical waves in terms of their need for a medium.
- Explain why sunlight can travel from the Sun to Earth while ordinary sound cannot travel through the same region of space.
- List the seven major regions of the electromagnetic spectrum in order from longest wavelength to shortest wavelength.
- Where is visible light located within the electromagnetic spectrum?
- Compare red and violet visible light in terms of wavelength and frequency.
- State the approximate speed of light in a vacuum in both m/s and km/s.
- A beam of light has a wavelength of 500 nm. Convert this wavelength into metres.
- Calculate the frequency of light with a wavelength of 500 nm using c = 3.00 × 10⁸ m/s.
- Explain why we normally see lightning before hearing thunder.
- Describe what happens to the speed, frequency, and wavelength of light when it travels from a vacuum into a transparent material where it moves more slowly.
- A student says, "Light must need air because we can see light travelling through the atmosphere." Explain why this reasoning is incorrect.
- Explain how observations of sunlight, starlight, reflection, refraction, diffraction, interference, and polarization provide evidence for the electromagnetic wave nature of light.