Emission Line Spectrum

The atom is first excited by a colliding electron. When the atom goes back to its ground state, either directly or via intermediate energy levels, photon of only certain frequencies are emitted due to the discrete energy levels. Hence only certain frequencies of light are observed, forming the emission spectrum, which is discrete bright coloured lines on a dark background.

 

Steps to obtain emission line spectrum:

  • Gases such as hydrogen or neon are placed in an discharge tube at low pressure.
  • A high voltage of several kilo-volts is applied across the cathode and anode of the discharge tube.
  • The gas atoms become excited by the collision with the electrons passing through the tube.
  • When the gas atoms fall to a lower energy level, the excess energy is emitted as electromagnetic radiation(photon) with a specific frequency. The frequency f of the emission line is dependent on the difference between the high and low energy levels. ΔE = hf
  • Only certain frequency lines are present in the spectrum as only certain high to low energy level transitions are possible within the atom.

 

The emitted light are analyzed with a spectrometer and discrete bright lines in a dark background are observed.

The well-defined separation of lines is experimental evidence for the existance of separate or ‘quantized’ energy levels in the atom. No two gases give the same exact line spectrum.

If the gases used are not at low pressure, there will be a continuous range of colours. At high pressure, tightly packed gas atoms or molecules will be vibrating, rotating or colliding with each other, such that many more energy levels will be created. Hence, there will be no separated/isolated lines of definite frequency.

 

emission spectrum of hydrogen

Emission Line Spectrum of hydrogen

 

Emission_spectrum of iron

Emission Line Spectrum of iron



Absorption Line Spectrum

White light is used to excite the atoms. Those incident photons whose energies are exactly equal to the difference between the atom’s energy levels are being absorbed. Since the energy levels are discrete, only photons of certain frequencies are absorbed. As these frequencies of light are now missing, they account for the dark lines in the absorption spectrum, which is discrete dark line on a continuous spectrum

 

Steps to obtain absorption line spectrum:

  • Gases such as hydrogen are placed in a tube.
  • White light is passed through the tube.
  • The atoms of the gas absorb light of the same wavelengths which they can emit, and then re-radiate the same wavelengths almost immediately but in all directions. Hence, the parts of the spectrum corresponding to these wavelengths appear dark by comparison with the other wavelengths not absorbed.

 

Spectral_lines_absorption

Absorption Line Spectrum of hydrogen



Distinguish between emission and absorption line spectra

In the case of an emission spectrum, the atom is first excited by a colliding electron. The colliding electron must have kinetic energy greater than or equal to the difference between energy levels of the atom. When the atom goes back to its ground state, either directly or via the intermediate energy levels, photon of only certain frequencies are emitted due to the discrete energy levels. Hence only certain frequencies of light are observed, forming the emission spectrum, which is discrete bright coloured lines on a dark background.

In the case of an absorption spectrum, white light is used to excite the atoms. Those incident photons whose energies are exactly equal to the difference between the atom’s energy levels are being absorbed. SInce the energy levels are discrete, only photons of certain frequencies are absorbed. As these frequencies of light are now missing, they account for the dark lines in the absorption spectrum, which is discrete dark line on a continuous spectrum.

 

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