Sunlight crosses 150 million kilometres of empty space to reach you, through a vacuum that would silence any sound. Light pulls off what sound cannot because it is a different kind of wave entirely: an electromagnetic wave that carries its own oscillating electric and magnetic fields and needs no medium at all. In a vacuum every such wave travels at the same enormous speed, $c$, the cosmic speed limit.
You learned that sound cannot cross the vacuum of space. Yet you can clearly see the Sun and distant stars.
Before reading: if light has no air, water or any material to travel through between the Sun and Earth, what could it possibly be "waving"? Write your best idea.
An electromagnetic wave carries its own fields
Light is an electromagnetic wave: instead of disturbing particles, it consists of an oscillating electric field and an oscillating magnetic field that keep regenerating each other as the wave moves, so it carries everything it needs and requires no medium.
Unlike sound, light is not a mechanical wave. It is an electromagnetic (EM) wave made of an oscillating electric field and an oscillating magnetic field, at right angles to each other and to the direction of travel, so it is a transverse wave. These changing fields continually create one another, allowing the wave to propagate through completely empty space. That is why starlight crosses the vacuum that would silence any sound. In a vacuum, all EM waves travel at the same speed, the speed of light $c = 3.0 \times 10^8\ \text{m s}^{-1}$.
Light is an electromagnetic wave made of oscillating electric and magnetic fields. It requires no medium and travels at the speed of light $c = 3.0 \times 10^8\ \text{m s}^{-1}$ in a vacuum.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: like sound, light needs a material medium to travel from the Sun to the Earth.
Accelerating charges make light; the wave carries energy
We just saw that light is made of oscillating fields. That raises a question: where do those fields come from, and how do they carry energy? This card answers it → accelerating charges produce them, and the wave transports energy as it travels.
An electromagnetic wave is launched whenever a charged particle accelerates, vibrating electrons in an aerial or a hot filament, and once launched the wave carries energy through space, depositing it when it is absorbed.
EM waves are produced by accelerating (vibrating) charges, for example, electrons oscillating in a radio aerial or in the hot atoms of a glowing filament. The acceleration sets up the oscillating electric and magnetic fields that propagate outward as the wave. As the wave travels it transfers energy: when it is absorbed by an object, that energy is delivered, warming your skin in sunlight, generating a current in a solar cell, or carrying a signal to an antenna. As always, the wave moves energy, not matter. Since light is a wave, the wave equation applies in the form $c = f\lambda$.
Green light has a frequency of $5.7 \times 10^{14}\ \text{Hz}$. Travelling through a vacuum at $c = 3.0 \times 10^8\ \text{m s}^{-1}$, what is its wavelength?
EM waves are produced by accelerating (vibrating) charges and transfer energy as they travel, delivering it when absorbed. As waves, they obey $c = f\lambda$ in a vacuum.
Add the highlighted statement to your notes before the check below.
Quick check: Electromagnetic waves are produced by:
Pattern A, Compare then justify
In your book make a comparison table of sound and light with rows for: (1) wave type (mechanical or electromagnetic); (2) needs a medium? (yes/no); (3) what is oscillating; (4) approximate speed; (5) can it cross the vacuum of space? Then write one sentence using your table to explain why we see lightning before we hear thunder.
Odd one out: Three of these statements about light as an electromagnetic wave are correct. Which one is the odd one out?
Dot point: Identify light as an electromagnetic wave that does not require a medium for propagation and travels at c, the speed of light, in a vacuum
Visible light is just the slice our eyes detect
We just learned how light is made and how it carries energy. That raises a question: is visible light the only kind of electromagnetic wave? This card answers it → no, it is one narrow band of a vast family.
The light your eyes see is only a thin slice of a much larger electromagnetic family that runs from radio waves to gamma rays, all of them the same kind of wave differing only in frequency and wavelength.
Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all electromagnetic waves. They differ only in frequency (and so wavelength), not in their fundamental nature, every one is an oscillating electric and magnetic field travelling at $c$ in a vacuum. Visible light is simply the narrow band our eyes happen to detect. Because all of them satisfy $c = f\lambda$ with the same $c$, a higher-frequency EM wave always has a shorter wavelength. The next lesson maps out this full electromagnetic spectrum.
Visible light is one band of the electromagnetic spectrum, which also includes radio, microwave, infrared, ultraviolet, X-ray and gamma waves. All are EM waves travelling at $c$ in a vacuum, differing only in frequency and wavelength.
Pause, copy the highlighted statement into your book before the check below.
Last check: Decide if the statement is true or false.
Radio waves and visible light are both electromagnetic waves and travel at the same speed in a vacuum.
Match them up: Match each stage in the production and transfer of an electromagnetic wave to its description.
Dot point: Describe the production and transfer of energy of electromagnetic waves
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 5(4 marks) 1. Explain why light can travel from the Sun to the Earth across empty space while sound cannot. Refer to the nature of each wave and how it is produced and transfers energy.
1 mark: sound is mechanical and needs a medium · 1 mark: light is an EM wave of oscillating electric and magnetic fields · 1 mark: EM waves need no medium and travel at $c$ in a vacuum · 1 mark: EM waves are produced by accelerating charges and transfer energy without matter
ApplyBand 4(3 marks) 2. Sunlight reaches Earth across roughly $1.5 \times 10^{11}\ \text{m}$ of near-empty space. Explain how this is possible for light but not for sound, and calculate the time the light takes to arrive.
1 mark: light is an electromagnetic wave and requires no medium · 1 mark: sound is mechanical and cannot cross a vacuum · 1 mark: $t = d/c = 1.5\times10^{11}/3.0\times10^8 = 500\ \text{s}$
Dot point: Identify light as an electromagnetic wave that does not require a medium for propagation and travels at c, the speed of light, in a vacuum
AnalyseBand 5(3 marks) 3. Describe how an electromagnetic wave is produced by an accelerating charge, and explain how the wave transfers energy to a distant receiver.
1 mark: an accelerating charge produces a changing electric field · 1 mark: the changing electric field induces a changing magnetic field, and the pair regenerate each other and propagate · 1 mark: at the receiver the fields exert forces on charges, transferring energy to them
Dot point: Describe the production and transfer of energy of electromagnetic waves
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): Sound is a mechanical wave, so it requires a medium of particles to propagate; the vacuum of space has no particles, so sound cannot cross it. Light is an electromagnetic wave, made of oscillating electric and magnetic fields that continually regenerate one another. Because the wave carries its own fields, it needs no medium and travels at the speed of light $c = 3.0 \times 10^8\ \text{m s}^{-1}$ in a vacuum. EM waves are produced by accelerating charges (such as vibrating electrons in the hot Sun) and transfer energy as they travel, energy that is delivered to the Earth when the light is absorbed, without transporting any matter across the gap.
Q2 (3 marks): Light is an electromagnetic wave. It propagates as a self-sustaining disturbance of electric and magnetic fields, which does not rely on particles being present, so it requires no medium and crosses the near-vacuum between the Sun and Earth unhindered. Sound is a mechanical wave: it propagates only by particles of a medium oscillating and colliding, so with essentially no particles present it cannot travel at all. The travel time follows from the speed of light in a vacuum: $t = d/c = (1.5 \times 10^{11}) / (3.0 \times 10^8) = 500\ \text{s}$, which is about $8.3$ minutes.
Q3 (3 marks): A stationary charge has a steady electric field and an unaccelerated charge a steady pattern of fields, but when a charge accelerates, for example when electrons oscillate up and down an aerial, it produces a changing electric field. A changing electric field induces a changing magnetic field, which in turn induces a changing electric field, and this mutual regeneration allows the disturbance to propagate away from the source at the speed of light without needing any medium. Energy is carried in these oscillating fields. When the wave reaches a distant receiver, its oscillating electric field exerts a force on the charges in the receiving aerial and drives them back and forth. Work is done on those charges, so energy is transferred from the wave to the receiver, and the resulting oscillating current is the received signal.
You can now identify light as an electromagnetic wave that needs no medium and travels at $c$, describe how accelerating charges produce EM waves, and explain how they transfer energy. Next lesson maps the full electromagnetic spectrum and uses $c = f\lambda$ and $E = hf$ quantitatively.
↩ Back to focus-area hubAt the start you wondered what light could be "waving" in empty space. The answer is its own electric and magnetic fields, no medium needed. That is exactly why starlight crosses the same vacuum that silences sound, and why all EM waves share the single speed $c$ in a vacuum.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
Tell us what you would like help with. Register interest; support details will be confirmed before enrolment.
Register interest →