The Mercury Spectrum
Introduction
A spectrometer is an instrument used for studying electromagnetic emissions. In this
experiment a spectrometer equipped with a diffraction grating is used to identify spe-
cific wavelengths from the emission spectrum of mercury, to measure these wave-
lengths precisely, and to compare them to accepted values.
Procedure
Adjust the diffraction grating so that the normal to its plane makes a small angle α
to the incident beam of light. This is shown schematically in Fig. 1. Since αu0,
the angles between the first and zeroth order intensity maxima on either side, θand θ0
respectively, are related to the wavelength λof the incident light according to [1]
λ=dsin(φ),(1)
accurate to first order in α. Here, dis the separation between the slits of the grating,
and
φ=θ0+θ
2(2)
Four visible spectral lines of mercury are depicted in Fig. 2. The accepted values
of their wavelengths and color associations are summarized in Table 1. To determine
the wavelengths of these spectral lines proceed as follows.
1. Turn on the power supply to which is attached the mecury discharge tube.
Color Wavelength [nm]
Violet 435.8
green 546.1
yellow–1 577.0
yellow–2 579.1
Table 1: A summary of wavelengths and color associations of the visible spectral lines
of mercury, as depicted in Fig. 2.
1
