In the first exercise we will observe the spectral lines of mercury and use the known wavelengths of these spectra to calibrate the grating spectrometer. In the second exercise, we will use the calibrated spectrometer to observe and measure the spectral lines of hydrogen. From the data we will make two independent determinations of the Rydberg constant. The diffraction grating provides the simplest and most accurate method for measuring wavelengths of light.
Transmission gratings are ruled on glass and the unruled areas transmit the incident light.
This experiment uses a transmission-grating replica. The slits of a grating give rise to diffraction and the diffracted light interferes so as to set up interference patterns. Complete constructive interference occurs when the phase or path difference is equal to some whole number of the wavelength. In general the grating equation for constructive maxima is where m is called the order of the spectrum, l is the wavelength, d is the spacing between grating lines, and J is the diffraction angle measured with respect to the direction of the light incident on the grating. Each gas emits a particular set of spectral lines and has its own characteristic spectrum.