Electromagnetic Waves | HL IB Physics Revision Notes 2025

The Electromagnetic Spectrum

An electromagnetic wave is generated by the combined oscillation of an electric and a magnetic field
These fields oscillate perpendicularly to each other and to the direction of motion of the wave (i.e. the direction in which energy is transferred)

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An electromagnetic wave is generated by the combined oscillation of an electric and a magnetic field

Electromagnetic waves are transverse waves and, as such, they can travel through vacuum
Regardless of their frequency, all electromagnetic waves travel at the speed of light c = 3 × 10⁸ m s^–1 in vacuum
Electromagnetic waves form a continuous spectrum based on their frequency (or wavelength)
The shorter the wavelength, or higher the frequency, the greater the energy of the wave

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The electromagnetic spectrum

Humans can only sense electromagnetic waves with wavelengths in the range 700 nm - 400 nm, which are the limits of the so-called visible spectrum
- Electromagnetic waves with longer and shorter wavelengths are invisible to the human eye

Knowing the wavelengths of electromagnetic waves, their frequencies can be calculated using
- The wave equation
- The fact that the speed of light (c = 3 × 10⁸ m s^–1) in a vacuum is constant

Worked example

:The wavelength of blue light falls within the range 450 nm – 490 nm.

Determine the range of frequencies of blue light.

Answer:

Step 1: Write down the known quantities

Note that you must convert the values of the wavelength from nanometres (nm) into metres (m)

- λ_lower = 450 nm = 4.5 × 10^–7 m
- λ_higher = 490 nm = 4.9 × 10^–7 m

Step 2: Remember that all electromagnetic waves travel at the speed of light in vacuum

From the data booklet, c = 3.00 × 10⁸ m s^–1

Step 3: Write down the wave equation

$v = f λ$

Step 4: Rearrange the above equation to calculate the frequency f

$f = \frac{v}{λ}$

Step 5: Substitute the lower and higher values of the wavelength to calculate the limiting values of the frequency of blue light

The lower frequency f_lower corresponds to the higher value of the wavelength λ_higher

$f_{l o w e r} = \frac{3.00 \times 10^{8}}{4.9 \times 10^{- 7}} = 6.1 \times 10^{14} Hz$

The higher frequency f_higher corresponds to the lower value of the wavelength λ_lower

$f = \frac{3.00 \times 10^{8}}{4.5 \times 10^{- 7}} = 6.7 \times 10^{14} Hz$

Step 6: Write down the range of frequencies of blue light

$f = 6.1 \times 10^{14} - 6.7 \times 10^{14} Hz$

Examiner Tip

You don't need to memorise the order or the wavelengths of waves in the electromagnetic spectrum, as this is given in your data booklet. However, you must remember all electromagnetic waves travel at the speed of light, c.

Comparing Mechanical & Electromagnetic Waves

Travelling waves can be of two types, mechanical and electromagnetic

Mechanical Waves	Electromagnetic Waves
Require a medium, such as a fluid or solid to propagate through	Do not require a medium
Can be transverse or longitudinal	Are only transverse
Cannot travel through a vacuum	Can travel through a vacuum
Are produced by the oscillation of particles in a medium	Are produced by oscillating charged particles
Examples: Sound waves, waves on the surface of the ocean	Examples: Radio waves, UV rays, X-rays
Travel a lot slower than the speed of light	Travel at the speed of light

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Electromagnetic waves can travel in a vacuum (such as space) whilst mechanical waves require a medium (such as water)

Examiner Tip

You must be able to identify the features of either mechanical or electromagnetic waves, depending on the question.

A common misconception is that mechanical waves are just longitudinal, like sound waves. However, they can also be transverse waves that travel through a medium such as seismic waves (created during an earthquake) and water waves.

Electromagnetic Waves (HL IB Physics)

Revision Note

The Electromagnetic Spectrum

Worked example

Examiner Tip

Comparing Mechanical & Electromagnetic Waves

Examiner Tip

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Author: Ashika