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

• 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

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

Step 1: List the known quantities

• • Wavelength,  λ = 588 nm = 5.88 × 10^–7 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 = 

 Step 3: Substitute the known values and calculate ΔE

 ΔE =  = 3.38 × 10^–19 J

 Step 4: Convert from J to eV

 ΔE =  = 2.11 eV

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