M7-S4: Applications of Spectroscopy

Discharge tubes are used to generate atomic emission spectra from gases

  • A discharge tube is a simple apparatus consisting of two metal electrodes connected to a high voltage potential difference. The metal electrodes are placed within an enclosed glass tube that contains a particular gas.

Conditions:

  • The high voltage (greater than 1000 V) is essential to permit the electric current to pass through air. This is achieved by producing a very strong electric field between the metal electrodes.
  • Gas discharge tubes contain low pressure to reduce the movement of gas molecules in air. Reduced movement leads to lower rate of molecular collision such that electrons’ motion (affected by the electric field) are less impeded. Overall, reduced pressure facilitates conduction of current in a gas medium. 

 

  • When electrons collide with gas molecules in the glass tube, their energy are partially transferred to the electrons in the gas atoms. If the energy is great enough, it will cause electron excitation from the ground state. Afterwards, electrons will emit this energy in the form of electromagnetic radiation and return to the ground state.
  • Depending on the identity of the gas, the emitted radiation may fall in the spectrum of visible light. This means the glass tube will radiate a particular colour. 

 

Reflected sunlight demonstrates absorption and emission spectra of molecules in the atmosphere

  • The spectrum of reflected sunlight consists of two main bumps – one from the visible spectrum and one from the infrared spectrum.
  • Usually, the electromagnetic spectrum is entirely continuous which supports Maxwell’s classical theory of electromagnetic radiation. However, through spectroscopy, reflected sunlight have discrete missing bands or lines (shown below)

  

  • Blue light, due to its higher level of energy compared with other waves of the visible light spectrum, is often absorbed in the atmosphere as the energy is sufficient to excite electrons in various molecules.
  • Carbon dioxide absorbs infrared radiation and long wavelengths of visible light. This is one of the main contributing factors of global warming because carbon dioxide reduces the ability of Earth’s surface in dissipating heat. Hence the name – greenhouse gas.
  • Upper atmosphere of Earth also contains molecules that emit UV radiation which is also captured in a collected sample of sunlight.

 

Light produced by incandescent filament lamps is continuous.

  • When electric current is passed through a conductor with very high resistance, the collision of electrons with the intrinsic structure of the conductor creates heat and light.
  • In incandescent filaments, the resistance is controlled such that it can generate as much light as possible without completely stopping the movement of the electric current.
  • Energy transformation: kinetic energy of electron (electrical energy) à heat energy & light energy.
  • Obviously, this method of generating light energy is very inefficient as heat is also produced.
  • However, the spectrum produced by incandescent filament is continuous as it is not a result of electronic excitation but production of actual light. The spectrum contains higher intensity of red light (lower energy) compared with blue light (higher energy) – see below for visualisation.

  

Diagram above for reference only, you do not need to know them for HSC Physics.

 


  • Explain what ‘stellar spectra’ is

Stars emit radiation which includes visible light (400 to 700 nm). For example, the Sun emits white light that can be captured and observed on a spectrometer. Instead of seeing a continuous spectrum, the stellar spectra of the Sun contain missing bands or gaps within the spectrum. This is referred to the absorption line spectrum.

 

The energy indicated by stellar spectra reveals information about stars’ surface temperature.

Radiation with longer wavelengths have lower energy. We can use this information to compare the relative temperature between stars. The Wien’s Law quantitatively describes this relationship:

 

Table below for reference only, in-depth knowledge is not required for Module 7. However, knowledge will be covered and required in Module 8.

Spectral Class

Colour

Surface temperature (K)

Elements evident in absorption lines

O

blue

over 30 000

ionised He, weak H

B

blue-white

30 000 – 15 000

neutral He, weak H

A

white

15 000 – 10 000

strong H

F

white-yellow

10 000 – 7 000

weak H, metals (Ca, Fe)

G

yellow

7000 - 5000

strong metals, esp. Ca

K

orange

5000 - 4000

strong metals; CH and CN

M

red

4000 - 3000

strong molecules (incl. TiO)

 

 

  • Stellar absorption lines are caused by elemental atoms in stars’ outer atmosphere.

When light is emitted by the star, some of its energy absorbed by atoms in the outer layers of its atmosphere. This occurs when the energy matches exactly the excitation energy of electrons in the ground state of these elements. The ‘missing’ spectral lines captured on Earth represent the energy that has been absorbed by these electrons. 

Stellar spectra can provide information on the chemical composition of stars.

  • Different stars are surrounded by atmospheres composed of different elements, atoms and ions. As such, we can use the spectral information to differentiate between different stars, and more importantly deduce their atmospheric composition.
  • By comparing the spectra with absorption line spectra, we can obtain on Earth using a wide range of elements such as hydrogen, helium and all the way up to iron, we can attain a pretty good idea of what elements are found in stars.
  • Using this method, helium was discovered on the sun before it was identified on Earth
  • The intensity of each spectral line also correlates with the relative abundance of each element.
  • However, it is important to note that the absence of spectral lines does not necessarily indicate the absence of a particular element. Excitation of electrons require specific physical conditions besides energy of light, which may not be present in certain stars.

 

Doppler’s Effect

  • The wavelength or frequency of a wave is influenced by the wave’s relatively velocity to the observer.
  • If the emitter or source of wave is moving towards the observer, the resultant wave has shorter wavelength and greater frequency.
  • If the emitter or source of wave is moving away from the observe, the resultant wave has longer wavelength and lower frequency.

 

  • Red and blue shifts observed in stellar spectra are caused by the Doppler’s Effect.

The following three spectra are for a hydrogen atom.

 

Red and blue shifts reveal information about stars’ translational and rotational motion.

The movement of stars, whether towards or away from Earth, is demonstrated by red and blue shift effects in their spectra. The shifting effect is apparent when stellar spectra are compared with absorption spectra obtained by exciting elements on Earth.

  • When stars move away from us, spectral lines of certain elements e.g. hydrogen would be shifted towards longer wavelengths compared with a reference.
  • When stars move towards us, spectral lines of certain elements would be shifted towards shorter wavelengths compared with a reference.

  

         This effect can also be used to determine the rotational behaviour of stars.

  • Wave emitted from the side of a star that is rotating towards us has shorter wavelength
  • Wave emitted from the side of a star that is rotating away from us has longer wavelength
  • Wave emitted from the ‘middle’ of a start (region that has no relative rotation to an observer on Earth) has no change in wavelength

 

The combined effect of these three phenomena results in broadened absorption lines in the stellar spectra

 

Width and shape of spectral lines also reveal information about the density of stars

    • Density and pressure, at the surface of a star can also broaden spectral lines, but the intensity varies across the line in different way from the effect of rotation.
    • In high density (small and massive) stars the increased gas pressure produces more rapid collision between atoms during the emission or absorption of radiation. These collisions cause changes in the electron orbits and hence produce a broader spectral line.

     

    Practice Question 1 (2 marks) 

    Explain what conditions are required to produce emission spectra using gas discharge tubes.

     

    Practice Question 2 (2 marks)

    When elements are not present in a star's absorption spectrum, it does not necessarily indicate that they are absent in the star's atmosphere. Explain why this might be so. 

     

    Practice Question 3 (4 marks) 

    Describe what information can be obtained from a star's absorption spectrum. Present your answer in a table. 

     

     

    Previous section: Introduction to Spectroscopy

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