The Electromagnetic Spectrum (DP IB Chemistry): Revision Note

The electromagnetic spectrum

• The electromagnetic spectrum is a range of frequencies that covers all electromagnetic radiation and their respective wavelengths and energy

• It is divided into bands or regions, and is very important in analytical chemistry.

• The spectrum shows the relationship between frequency (f), wavelength (λ) and energy

• Frequency is how many waves pass per second, and wavelength is the distance between two consecutive peaks on the wave

• Gamma rays, X-rays, and UV are high frequency, high energy:

  • These can damage cells and are hazardous to health

The electromagnetic spectrum diagram

• All electromagnetic waves travel at the same speed in a vacuum

• This speed is the speed of light (c) = 3.00 × 10⁸ m s⁻¹

• What differs between waves is their frequency (f) and wavelength (λ)

• Frequency and wavelength are inversely related:

  • Higher frequency means a shorter wavelength

  • Lower frequency means a longer wavelength

• The key relationship is:

c=fλ

• Since c is constant, you can calculate frequency from wavelength, or vice versa

Continuous versus line spectrum

• A continuous spectrum in the visible region contains all colours of visible light

• It is produced when white light is refracted through a prism or water droplets (as in a rainbow)

• The colours blend smoothly without any gaps in frequency or wavelength

Continuous spectrum diagram

• However, a line spectrum only shows certain frequencies

Helium spectrum diagram

• This tells us that the emitted light from atoms can only be certain fixed frequencies - it is quantised (quanta means 'little packet')

• Electrons can only possess certain amounts of energy - they cannot have any energy value