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Index of refraction, dipersion, and resolving power of a prism spectroscope

Index of refraction, dipersion, and resolving power of a prism spectroscope

Item no.: P2210500

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The spectrometer-goniometer is used to determine the angle of deflection that light undergoes when passing a prism. The refractive index of a prism at the specific wavelength of a known spectral line can be determined by observation of the angle of least deflection of light of that spectral line effected by that prism, given that the angle between the planes of the prism is known. If this measurement is done for different spectral lines, the dispersion curve of the prism can be determined. Prisms made of different glass and a hollow prism that can be filled with different liquids are used to determine dispersion curves of different types of glass and different liquids. Further can be shown how the spectral resolving power of a prism spectrometer depends on the dispersion and size of a prism. For this an adjustable slit is used to restrict the useful width of the light beam. The effect on the resolving power of a restricted beam width is observed.



  • understand fundamentals of spectroscopy
  • compact setup
  • precise and reproducible results


  1. Set up the Goniometer as grating spectrometer and identify the spectral lines of the Hg spectral lamp with help of a grating and assign the corresponding wavelengths to the visible lines.
  2. Select one of the following equilateral (60°) prisms: glass BK7, glass F2, glass SF10, hollow prism filled with one of glycerol, 2-propanol, water, and adjust the gioniometer as prism spectrometer.
  3.  Optional: Use the spectrometer-goniometer to measure the angle between the surfaces effecting the refraction of the prism in use by observing the surface reflexes and turning the prism table with fixed angle between collimating and observing telescope. 
  4.  Measure the angle of least deflection for each of the well visible Hg spectral lines for the prism in . 
  5. From the angle of least deflection calculate the refractive index and plot that over the wavelength. 
  6. Use a slit to reduce the free beam path and observe the slit width at which specific lines can no longer be separated. From the plot of refractive index over wavelength determine the dispersion at these wavelenghts and compare theoretical with observed resolving power of the spectroscope.

What you can learn about

  • Maxwell relationship
  • Dispersion
  • Polarisability
  • Refraction, Refractive index
  • Prism
  • Rowland grating
  • Spectrometer
  • Goniometer
  •  Dielectric polarizability
  • Relative electric and magnetic permissivity
Document     Filesize
p2210300.pdf Experiment guide, English 465.36 KB

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