Determine the Rydberg constant by studying Hydrogen spectrum


To determine the Rydberg constant by studying Hydrogen spectrum.


Balmer series is the emission spectrum associated with the transition of electrons from states with the principal quantum number n>2 to the state with n = 2. The wavelengths of the corresponding lines are expressed by a formula

where, n1 = 2 and n2 > 2.

In Hydrogen spectrum, there are a few bright lines, namely

Hα : n2 = 3 : Colour = Red

Hβ : n2 = 4 : Colour = Blue-green

Hγ : n2 = 5 : Colour = Violet

Hδ : n2 = 6 : Colour = Violet

In the present experiment wavelengths of these lines are determined by measuring the angle of diffraction by a transmission grating using the formula

Here α is the width of the opaque line in the grating and b is the width of the transparent region between two opaque lines and consequentlywhere N is the number of rulings per unit length of the grating, n is the order of the spectrum and θ is the angle of diffraction.


Spectrometer, Spirit level, Hydrogen discharge tube, Power supply for the discharge tube. (A magnifying lens and a torch in hand may prove useful)

Experimental results:

  • Vernier Constant of the Spectrometer: 
  • Measurement of angle of Diffraction:


No. of Rulings N = ………………. per mm.=……………… per cm.


λred                  =

λblue-green  =

λviolet-1 =

λviolet-2  =

RH from red line=

RH from blue-green line  =

RH from violet-1 line=

RH from violet-2 line =

Average RH =

Standard value of RH   =

Percentage Error:



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