IB Chemistry SL

Revision Notes

2.1.4 The Electromagnetic Spectrum

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, wavelength 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 radiation are all dangerous – you can see from that end of the spectrum that it is high frequency and high energy, which can be very damaging to your health

EM spectrum diagram, downloadable AS & A Level Physics revision notes

  • All light waves travel at the same speed; what distinguishes them is their different frequencies
  • The speed of light (symbol ‘c’) is constant and has a value of 3.00 x 108 ms1
  • As you can see from the spectrum, frequency (symbol ‘ν‘) is inversely proportional to wavelength (symbol ‘λ‘)
    • In other words, the higher the frequency, the shorter the wavelength
  • The equation that links them is c = νλ 
  • Since c is constant you can use the formula to calculate the frequency of radiation given the wavelength, and vice versa

Continuous versus line spectrum

  • A continuous spectrum in the visible region contains all the colours of the spectrum
  • This is what you are seeing in a rainbow, which is formed by the refraction of white light through a prism or water droplets in rain

A continuous spectrum shows all frequencies of light

  • However, a line spectrum only shows certain frequencies

The line spectrum of helium which shows only certain frequencies of light

  • 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

Exam Tip

The formula that relates frequency and wavelength is printed in Section 1 of the IB Chemistry Data Booklet so you don’t need to learn it

You will also find the speed of light and other useful constants in Section 2

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