13.1 The Electromagnetic Spectrum

A free GCSE Physics lesson for AQA (8463) from OBA School: electromagnetic waves, the order of the spectrum, visible light and the wave speed equation. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe electromagnetic waves as transverse waves that transfer energy from a source to an absorber
• List the parts of the electromagnetic spectrum in order of wavelength or frequency
• State that all electromagnetic waves travel at the same speed, 3.0 × 10⁸ m/s, in a vacuum
• State the range of wavelengths that the human eye can detect
• Use v = f λ, with standard form and unit prefixes, to calculate frequency or wavelength

Key words

• EM wave: A transverse wave made of linked electric and magnetic oscillations. It transfers energy, not matter, and can travel through a vacuum.
• Transverse: A wave in which the oscillations are at right angles (90°) to the direction the wave transfers energy.
• EM spectrum: The continuous range of EM waves, from long-wavelength radio waves to short-wavelength gamma rays.
• Wavelength, λ: The distance from one point on a wave to the same point on the next wave, e.g. crest to crest. Unit: metre (m).
• Frequency, f: The number of waves passing a point each second. Unit: hertz (Hz); 1 Hz = 1 wave per second.
• Continuous: Having no gaps: the parts of the spectrum merge into each other, so the boundaries are only approximate.

What electromagnetic waves are

Electromagnetic waves are transverse waves. They are made of electric and magnetic fields that oscillate at right angles to the direction the wave travels. Unlike sound or water waves they need no medium, so they can cross empty space – that is how energy from the Sun reaches the Earth.

EM waves transfer energy from a source (the object that emits them) to an absorber (the object that takes them in). They do not transfer matter. For example, a toaster’s glowing elements are the source of infrared radiation and the bread is the absorber, which heats up.

Key fact: In a vacuum (and almost exactly in air) every EM wave travels at the same speed: 3.0 × 10⁸ m/s (300 000 000 m/s). Gamma rays are not faster than radio waves.

The electromagnetic spectrum

EM waves are grouped by wavelength (or frequency) into seven families. The spectrum is continuous: one family blends into the next, so the wavelengths at the boundaries are only rough guides.

• Long wavelength, low frequency, low energy: radio waves (longest), microwaves, infrared.
• Visible light sits in the middle.
• Short wavelength, high frequency, high energy: ultraviolet, X-rays, gamma rays (shortest).

Exam tip: Make up a phrase to remember the order, e.g. Rabbits Might Invade Vegetable Units eXpecting Greens: radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma.

Because the speed is fixed, a shorter wavelength always means a higher frequency. Higher-frequency waves carry more energy, which is why ultraviolet, X-rays and gamma rays can damage living cells while radio waves are much safer.

EM waves are made or absorbed when particles change energy. Electrons moving between energy levels in atoms can emit or absorb infrared, visible light and ultraviolet; oscillating charges in electric circuits produce radio waves; gamma rays come from changes in the nucleus of an atom.

Visible light

Your eyes can detect only a very small part of the spectrum: visible light, with wavelengths from about 400 nm (violet) to 700 nm (red). (1 nm = 1 nanometre = 1 × 10⁻⁹ m.) Each wavelength gives a different colour; white light is a mixture of all of them.

Different materials absorb, transmit, reflect or refract EM waves in different ways, depending on the wavelength. Glass transmits visible light but absorbs much of the ultraviolet; the atmosphere lets visible light and most radio waves through but absorbs most ultraviolet and X-rays from space.

Calculating with v = f λ

wave speed = frequency × wavelength v = f λ

v = wave speed in metres per second (m/s); f = frequency in hertz (Hz); λ = wavelength in metres (m)

Rearranged: f = v ÷ λ λ = v ÷ f For EM waves in a vacuum or air, v = 3.0 × 10⁸ m/s

• kilo, k; Meaning × 10³; Example 540 kHz = 5.4 × 10⁵ Hz
• mega, M; Meaning × 10⁶; Example 95.8 MHz = 9.58 × 10⁷ Hz
• giga, G; Meaning × 10⁹; Example 2.4 GHz = 2.4 × 10⁹ Hz
• milli, m; Meaning × 10⁻³; Example 6.0 mm = 6.0 × 10⁻³ m
• micro, µ; Meaning × 10⁻⁶; Example 10 µm = 1.0 × 10⁻⁵ m
• nano, n; Meaning × 10⁻⁹; Example 480 nm = 4.8 × 10⁻⁷ m

Worked example: wavelength of a Wi-Fi signal

A Wi-Fi router sends signals at a frequency of 5.0 GHz. Calculate the wavelength.
f = 5.0 GHz = 5.0 × 10⁹ Hz
λ = v ÷ f = 3.0 × 10⁸ ÷ 5.0 × 10⁹ = 0.060 m

Answer: λ = 0.060 m (6.0 cm)

Worked example: frequency of green light

Green light from a laser pointer has a wavelength of 520 nm. Calculate its frequency.
λ = 520 nm = 520 × 10⁻⁹ m = 5.2 × 10⁻⁷ m
f = v ÷ λ = 3.0 × 10⁸ ÷ 5.2 × 10⁻⁷ = 5.77 × 10¹⁴ Hz

Answer: f = 5.8 × 10¹⁴ Hz (2 s.f.)

Exam tip: Convert prefixes to plain units (Hz and m) before you use the equation. On a calculator, type 3.0 × 10⁸ as 3.0 [×10ˣ] 8 – do not type “× 10” separately.

Because EM waves have a fixed speed, you can also find how long they take to travel a distance with distance = speed × time. Sunlight travels 1.5 × 10¹¹ m to Earth, so it takes 1.5 × 10¹¹ ÷ 3.0 × 10⁸ = 500 s (just over 8 minutes).

Common mistakes

• Saying that high-energy waves such as gamma rays travel faster. All EM waves have the same speed in a vacuum.
• Mixing up the directions: wavelength increases towards radio waves; frequency and energy increase towards gamma rays.
• Forgetting to convert MHz, GHz or nm into Hz and m before using v = f λ.
• Calling EM waves longitudinal. They are transverse, like ripples on water, not like sound.
• Saying EM waves carry matter or particles from place to place. They transfer energy only.

Revision questions

Try these, then check your answers below each one.

Question 1. Which list puts these electromagnetic waves in order of increasing wavelength? (Choose from: gamma rays, ultraviolet, infrared, microwaves, microwaves, infrared, ultraviolet, gamma rays, ultraviolet, gamma rays, microwaves, infrared, gamma rays, infrared, ultraviolet, microwaves.)

Answer: gamma rays, ultraviolet, infrared, microwaves

Working: Wavelength increases from gamma rays → X-rays → ultraviolet → visible → infrared → microwaves → radio waves, so the order is gamma rays, ultraviolet, infrared, microwaves.

Question 2. True or false? In a vacuum, X-rays travel faster than radio waves.

Answer: False

Working: False. All electromagnetic waves travel at the same speed in a vacuum, 3.0 × 10⁸ m/s. X-rays carry more energy, but they are not faster.

Question 3. A radio station broadcasts at 102 MHz. The speed of radio waves is 3.0 × 10⁸ m/s. Calculate the wavelength. Give your answer to 3 significant figures.

Answer: 2.94 m

Working: f = 102 × 10⁶ Hz = 1.02 × 10⁸ Hz. λ = v ÷ f = 3.0 × 10⁸ ÷ 1.02 × 10⁸ = 2.94 m.

Related lessons

• Previous lesson: 12.8 Required Practical: Waves
• Next lesson: 13.2 Light, Infrared, Microwaves and Radio Waves
• In this chapter: 13.3 Communications
• In this chapter: 13.4 Ultraviolet Waves, X-rays and Gamma Rays
• In this chapter: 13.5 X-rays in Medicine

Continue in the full course

This lesson is section 13.1 (AGCPH1301) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.