๐ Light & Sound Waves: Riding the Ripples of Energy
Explore how light and sound travel as waves, learn the key quantities that describe them, and use the wave equation to connect speed, frequency, and wavelength in everyday situations.
Learning Goals
By the end of this lesson, you will be able to:
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Describe light and sound as waves and identify their key properties (wavelength, frequency, amplitude).
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Apply the wave equation v=fฮป to calculate speed, frequency, or wavelength.
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Compare how light and sound travel through different materials and explain why their speeds differ.
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Relate wave properties to observable effects such as loudness, pitch, brightness, and colour.
Before You Begin
Before this lesson, you should know:
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Understand that energy can be transferred from one place to another.
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Be able to multiply and divide decimals and use units correctly.
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Know that sound needs a material (solid, liquid, gas) to travel through.
Notation & Symbols
Key notation and symbols used in this topic:
v
Wave speed, measured in metres per second (m/s).
f
Frequency โ number of waves passing a point per second, measured in hertz (Hz).
ฮป
Wavelength โ the length of one complete wave, measured in metres (m).
T
Period โ the time for one complete wave to pass, measured in seconds (s), where T=f1โ.
Hz
Hertz โ the unit of frequency, equal to one wave (cycle) per second.
What Are Light and Sound Waves?
A wave is a disturbance that transfers energy from one place to another without transferring matter. Both light and sound travel as waves, but they are different kinds of waves.
Transverse vs. longitudinal: Light is a transverse wave โ the vibrations are at right angles to the direction of travel. Sound is a longitudinal wave โ the vibrations are back-and-forth along the direction of travel, forming compressions and rarefactions in the air.
Wavelength ฮป: The distance between two matching points on a wave, such as crest to crest. A shorter wavelength of light looks more blue; a shorter wavelength of sound gives a higher pitch.
Frequency f: How many waves pass a point each second, measured in hertz. Higher frequency sound is higher pitched; higher frequency light is bluer or more energetic.
Amplitude: The maximum size of the vibration. Bigger amplitude means louder sound or brighter light.
Medium matters: Sound needs a material to travel through and cannot pass through a vacuum, while light can travel through empty space. This is why we see the Sun but hear nothing from space.
Example
A sound wave in air has a frequency of 200 Hz and a wavelength of 1.7 m. Using v=fฮป: v=200ร1.7=340m/s which matches the speed of sound in air.
100%
The top shows a transverse light wave with crests, troughs, and wavelength; the bottom shows a longitudinal sound wave with compressions and rarefactions.
Worked Example
Basic
A sound wave in air has a frequency of 170 Hz and travels at 340 m/s. What is its wavelength?
Worked Example
Intermediate
You see lightning and hear the thunder 6 s later. If sound travels at 340 m/s and light is effectively instant, how far away is the storm?
Worked Example
Advanced
A light wave of frequency 5ร1014 Hz travels at 3ร108 m/s in air. Find its wavelength, and compare the speed of light to the speed of sound (340 m/s) as a ratio.
Why It Works
Understanding the reasoning behind the rule:
The wave equation v=fฮป links the three key properties of any wave. Here is why it must be true.
Your Turn
Try this one yourself โ use the hint if you get stuck:
A water wave at the local pond has a wavelength of 0.5 m and a frequency of 4 Hz. What is its speed?
Watch Out For...
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Thinking sound can travel through empty space (a vacuum) like light does.
Sound is a mechanical wave and needs a medium (solid, liquid, or gas) to travel. Light is an electromagnetic wave and can travel through a vacuum, which is why we see the Sun but hear nothing from space.
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Confusing frequency and wavelength โ assuming higher frequency means longer wavelength.
For a fixed wave speed, frequency and wavelength are inversely related: v=fฮป. If frequency increases, wavelength must decrease.
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Mixing up amplitude with frequency when describing loudness and pitch.
Amplitude controls loudness (bigger amplitude = louder); frequency controls pitch (higher frequency = higher pitch). They are independent properties.
Extended Practice
Test yourself with a mix of multiple choice and short-answer questions:
Q1.
Which property of a sound wave determines how loud it sounds?
MCQ
Q2.
A wave has frequency 50Hz and wavelength 6m. What is its speed?
MCQ
Q3.
Explain why we see a firework explode before we hear it.
Light travels at about 3ร108m/s, far faster than sound at about 340m/s. So the light reaches our eyes almost instantly while the sound arrives moments later.
Q4.
A sound wave in air travels at 340m/s with a frequency of 170Hz. Find its wavelength.
ฮป=fvโ=170340โ=2m.
Q5.
Which wave can travel through a vacuum?
MCQ
Extension: Echoes and Distance
Advanced
A harder challenge that connects to the next skill:
You stand facing a cliff and shout. You hear the echo 1.5 seconds later. If the speed of sound in air is 340m/s, how far away is the cliff?
Key Takeaways
1.Waves transfer energy without transferring matter; sound is mechanical (needs a medium) while light is electromagnetic (travels through a vacuum).
2.The wave equation v=fฮป links speed, frequency, and wavelength.
3.Amplitude determines loudness/brightness; frequency determines pitch/colour.
4.Light travels much faster than sound, which explains everyday delays like thunder after lightning.
Printable Cutouts
Matched to this lesson โ untick anything you don't need, then print.
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Ready to Practice?
Now that you've learned Light & Sound Waves, put it to the test with a quick quiz!