Continuous, Emission Line & Absorption Line Spectrum (OCR A Level Physics)

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Katie M


Katie M



Continuous, Emission Line & Absorption Line Spectrum

  • There are three kinds of light spectra:
    • Continuous emission spectra
    • Emission line spectra
    • Absorption line spectra

Continuous Line Spectra

  • Photons emitted from the core of a star contain all the wavelengths and frequencies of the electromagnetic spectrum
    • This is called a continuous spectrum
  • Continuous spectra are produced from hot, dense sources, such as the cores of stars

Continuous Visible Spectrum

A continuous spectrum is one which emits all wavelengths of light


Emission Line Spectra

  • When an electron transitions from a higher energy level to a lower energy level, this results in the emission of a photon
    • Each transition corresponds to a different wavelength of light and this corresponds to a line in the spectrum
  • The resulting emission spectrum contains a set of discrete wavelengths
    • It is represented by coloured lines on a black background
  • Emission line spectra are produced by hot, low pressure gases


The emission spectrum of hydrogen

Absorption Spectra

  • An atom can be raised to an excited state by the absorption of a photon
  • When white light passes through a cool, low pressure gas it is found that light of certain wavelengths are missing
    • This type of spectrum is called an absorption spectrum

  • An absorption spectrum consists of a continuous spectrum
    • This is represented by a background of all the colours with dark lines at certain wavelengths
    • These dark lines correspond exactly to the differences in energy levels in an atom
  • When these electrons return to lower levels, the photons are emitted in all directions, rather than in the original direction of the white light
    • Therefore, some wavelengths appear to be missing

  • The wavelengths missing from an absorption spectrum are the same as their corresponding emission spectra of the same element


The absorption spectrum of hydrogen



Worked example

An absorption line at a wavelength of 588 nm is observed in the spectrum from a star.

Determine the difference between the energy levels for the atoms in the gas responsible for this absorption line. Give your answer in electron volts.     

Step 1: List the known quantities

    • Wavelength,  λ = 588 nm = 5.88 × 107 m
    • Planck’s constant, h = 6.63 × 10–34 J s
    • Speed of light, c = 3.00 × 108 m s–1

 Step 2: Identify the appropriate equation 

 ΔE = fraction numerator h c over denominator lambda end fraction

 Step 3: Substitute the known values and calculate ΔE

 ΔE = fraction numerator open parentheses 6.63 blank cross times blank 10 to the power of negative 34 end exponent close parentheses blank cross times blank open parentheses 3.00 blank cross times blank 10 to the power of 8 close parentheses over denominator 5.88 blank cross times blank 10 to the power of negative 7 end exponent end fraction  = 3.38 × 1019 J

 Step 4: Convert from J to eV

 ΔE = fraction numerator 3.38 blank cross times blank 10 to the power of negative 19 end exponent over denominator 1.60 blank cross times blank 10 to the power of negative 19 end exponent end fraction  = 2.11 eV


Worked example

Explain why:

Hot, dense sources produce continuous spectra
Hot, low pressure gases produce emission spectra
Hot, dense sources observed through cool, low pressure gases produce absorption spectra

Part (a)

Hot, dense sources, such as the cores of stars, produce continuous spectra because:

    • In a hot, dense material, the atoms or molecules are so close together that they interact with one another
    • This leads to a spread of energy states that are not clearly defined
    • Therefore, photons of all frequencies are emitted leading to an uninterrupted band of colour 

Part (b)

Hot, low pressure gases produce emission line spectra, because:

    • Hot gases produce emission line spectra when photons are emitted due to the transition of electrons
      between discrete energy levels in atoms of the gas
    • The line spectrum has certain, fixed frequencies related to the differences in energy between the various energy levels of the atoms of the gas
    • In a low pressure gas, the atoms or molecules are not close together
    • This means the energy levels of the gas atoms or molecules are clearly quantised and well-defined
    • Therefore, only photons which correspond to the differences in energy between the energy levels of a bound electron are seen

Part (c)

Hot, dense sources observed through cold gases produce absorption spectra because: 

    • Atoms of different elements in the cold gas absorb energy emitted from the hot source but only at particular energy values
    • These particular energy values correspond to the differences in energy between the energy levels of a bound electron
    • This means that particular frequencies of light are absorbed, creating black lines in the continuous emission spectrum

Exam Tip

You will be required to identify the part of the electromagnetic spectrum to which an absorption or an emission line belongs. To help with this, make sure you are familiar with the wavelength values.

You should also be able to determine the colour by knowing the wavelength when the absorption or emission lines are in the visible range.

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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