Science · Physics foundations
Waves: sound & light
Wavelength, frequency, and the spectrum.
What are waves?
Waves transfer energy from one place to another without moving matter along. Sound and light both travel as waves. Sound needs a material (like air or water) to travel through. Light can travel through empty space. This makes light special among all waves we know.
Wavelength and frequency
Wavelength is the distance between one wave peak and the next peak, measured in metres. Frequency is how many complete waves pass a point each second, measured in hertz (Hz). High frequency means short wavelength. Low frequency means long wavelength. They are connected by a simple rule.
The wave equation
Wave speed equals frequency multiplied by wavelength. Write it as: speed = frequency × wavelength. For sound in air, the speed is about 340 metres per second. For light in air or space, the speed is about 300,000,000 metres per second. Use this equation to find missing values.
The electromagnetic spectrum
Light is one type of electromagnetic wave. The whole family includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. They all travel at the same speed in space. They differ in wavelength and frequency. Red light has the longest wavelength we can see; violet the shortest.
Using different waves
Radio waves carry TV and phone signals because their long wavelengths travel far. Microwaves heat food by making water molecules vibrate. Infrared keeps us warm from heaters and the Sun. Ultraviolet causes sunburn and helps make vitamin D. X-rays pass through soft tissue to image bones.
Worked examples
1. A sound wave has a frequency of 500 Hz and travels at 340 m/s. What is its wavelength?
- Write the wave equation: speed = frequency × wavelength.
- Rearrange to find wavelength: wavelength = speed ÷ frequency.
- Substitute the values: wavelength = 340 ÷ 500 = 0.68 m.
2. Red light has a wavelength of 0.0000007 m and travels at 300,000,000 m/s. What is its frequency?
- Write the wave equation: speed = frequency × wavelength.
- Rearrange to find frequency: frequency = speed ÷ wavelength.
- Substitute the values: frequency = 300,000,000 ÷ 0.0000007 = 428,000,000,000,000 Hz.
3. Which has more energy: a radio wave with wavelength 3 m or ultraviolet with wavelength 0.0000003 m?
- Remember that shorter wavelength means higher frequency.
- Higher frequency means more energy carried by each wave.
- Ultraviolet has a much shorter wavelength, so it carries more energy.
Try it yourself
A wave has a frequency of 200 Hz and a wavelength of 1.5 m. Calculate its speed.
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