All light — from radio waves to gamma rays — is the same kind of energy, just at different frequencies. Understanding how frequency, wavelength and energy connect sets up everything that follows about how electrons absorb and emit light.
📘 What you need to know
The electromagnetic (EM) spectrum is the full range of frequencies of EM radiation, each with its own wavelength and energy.
It runs from low-energy, long-wavelength radio waves to high-energy, short-wavelength gamma rays.
All EM waves travel at the speed of light (c) = 3.00 × 108 m s-1 in a vacuum.
Frequency and wavelength are inversely related: c = fλ.
A continuous spectrum shows all frequencies blending; a line spectrum shows only certain fixed frequencies — evidence that energy is quantised.
What the spectrum shows
The EM spectrum is split into bands — radio, microwave, infrared, visible, ultraviolet, X-ray and gamma — arranged by their frequency, wavelength and energy.
Frequency (f) is how many waves pass a point each second.
Wavelength (λ) is the distance between two neighbouring peaks.
Gamma rays, X-rays and UV sit at the high-frequency, high-energy end — energetic enough to damage cells, so they can be hazardous to health.
The EM spectrum runs from long-wavelength radio waves to short-wavelength gamma rays.
Linking frequency and wavelength
Every EM wave travels at the same speed in a vacuum — the speed of light. Because that speed is fixed, frequency and wavelength are locked together: if one goes up, the other must come down.
Speed of light relationship
c = f λ
Higher frequency → shorter wavelength.
Lower frequency → longer wavelength.
Since c is constant, you can always find one from the other.
You don’t need to memorise this formula — c = fλ and the value of the speed of light are both given in the IB Chemistry data booklet (Sections 1 and 2). Just know how to use them.
Continuous vs line spectra
How light is spread out reveals something deep about energy.
A continuous spectrum contains all the colours of visible light, blending smoothly with no gaps — like a rainbow made when white light passes through a prism or water droplets.
A line spectrum shows only a few specific frequencies — bright lines separated by darkness.
A continuous spectrum blends every colour; a line spectrum shows only fixed frequencies.
The fact that atoms give out only certain frequencies is a huge clue: it means electrons can only have fixed amounts of energy, not any value they like. This idea — that energy comes in packets — is called quantisation (a “quantum” is a little packet of energy).
💡 Exam tip
Remember the direction of the trend: as you move from radio waves to gamma rays, frequency and energy rise while wavelength falls.
A line spectrum is the key evidence that electron energies are quantised — a favourite exam point.
Up next: Atomic Emission Spectra — what happens to electrons when they absorb and release energy, and why each element produces its own unique set of lines.
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