Electromagnetic Spectrum (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

Written by: Leander Oates, Physics

Reviewed by: Caroline Carroll, Physics Subject Lead

Electromagnetic Spectrum

Electromagnetic waves are defined as:

Transverse waves that transfer energy from the source of the waves to an absorber

All electromagnetic waves share the following properties:
- They are transverse
- They can travel through a vacuum
- They travel at the same speed in a vacuum
- They transfer energy
There are 7 types of electromagnetic waves, grouped by energy, wavelength and frequency, which together form a continuous spectrum
The electromagnetic spectrum is arranged in a specific order based on the wavelengths or frequencies
The order of the groups within the electromagnetic spectrum, from the lowest frequency (longest wavelength), is:
- Radio waves
- Microwaves
- Infrared
- Visible light
- Ultraviolet
- X-rays
- Gamma rays
The wavelengths range from radio waves with a wavelength of $1 \times 10^{4}$ m to gamma rays with a wavelength of $1 \times 10^{- 15}$ m and beyond

The electromagnetic spectrum

EM spectrum, for IGCSE & GCSE Physics revision notes — **The electromagnetic spectrum in order of decreasing wavelength and increasing frequency**

The relationship between the frequency and wavelength of waves across the electromagnetic spectrum is:
- The higher the frequency, the shorter the wavelength
- The lower the frequency, the longer the wavelength
This means that radio waves have a lower frequency and a longer wavelength than UV waves
This can be seen from the wave equation

$v = f \times λ$

Where:
- $v$ = speed of the wave in metres per second (m/s)
- $f$ = frequency of the wave in hertz (Hz)
- $λ$ = wavelength of the wave in metres (m)
Since all electromagnetic waves travel at the speed of light in a vacuum, speed is constant
- Therefore, in the equation, in keeping v constant, if f increases then λ must decrease

Relationship between wavelength and frequency

Frequency and wavelength, for IGCSE & GCSE Physics revision notes — **The larger the wavelength, the lower the frequency. The speed remains the same**

The higher the frequency, the more energy the wave transfers

Worked Example

One region of the electromagnetic spectrum has wavelengths in the range $1 \times 10^{- 11} m$ to $1 \times 10^{- 9} m$ . The wave speed of electromagnetic waves is $3 \times 10^{8} m / s$ .

Calculate the maximum frequency of this region of the electromagnetic spectrum.

Answer:

Step 1: Deduce the wavelength which would have the maximum frequency within this range

The maximum frequency will have the shortest wavelength
The shortest wavelength in this region is $1 \times 10^{- 11} m$

Step 2: List the known quantities

Wavelength, $λ = 1 \times 10^{- 11} m$
Wave speed, $v = 3 \times 10^{8} m / s$

Step 3: Write out the equation relating wave speed, frequency and wavelength

$ν = f \times λ$

Step 4: Rearrange for frequency and calculate the answer

$f = \frac{ν}{λ}$

$f = \frac{3 \times 10^{8}}{1 \times 10^{- 11}}$

$f = 3 \times 10^{19} Hz$

Examiner Tip

If you are not sure which wavelength would give the maximum frequency, you could calculate the frequency for both the shortest and longest wavelength to see which one gives the maximum value.

Visible Light

Visible light is one small part of the electromagnetic spectrum
It is the part that we can detect with our eyes
The colours that our eyes see are dependent on the wavelength (or frequency) of the detected light
- Red light has the longest wavelength and lowest frequency
- Violet light has the shortest wavelength and highest frequency

The spectrum of visible light

Spectrum of visible light, for IGCSE & GCSE Physics revision notes — **Colours of the visible spectrum with increasing wavelength**

Wavelength and frequency are inversely proportional, this means that:
- An increase in wavelength is a decrease in frequency (towards the red end of the spectrum)
- A decrease in wavelength is an increase in frequency (towards the violet end of the spectrum)

Last updated: 17 April 2024

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