H-alpha Isotope Shift

THIS EXPERIMENT IS MANDATORY FOR ALL STUDENTS

The goals of this experiment are to measure the wavelengths of the Balmer lines of hydrogen using a reflection grating spectroscope, and to obtain the wavelength shift between hydrogen and deuterium in the H-alpha line.

According to quantum mechanics, the ratio of emission wavelengths of hydrogen and deuterium is the inverse of the ratio of their reduced masses. The shift is very small: the ratio of wavelengths for a H/D spectral line is about 1.00027.

The spectrometer consists of a slit and collimater tube, a reflection grating, and a viewing telescope. The relative angles of each can be varied to establish angles of incidence and reflection from a light source. After using a mercury light source to determine the ruling spacing of the grating, students observe the Balmer emission lines in a mixed hydrogen-deuterium discharge tube. Measured wavelengths are used to determine the Rydberg constant. Finally, the geometry is optimized to quantify the isotope splitting of the H-alpha line.

For more information on this experiment, see the lab handout.

Photo of spectrometer and light source

This page was prepared by Grant Coble (fall '97), modified by K.M.

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