The incident beam of light is diffracted by the rulings on the grating and emission lines can be viewed along the meterstick, on either side of the incident beam, as indicated by observers 1 and 2 in Figure 4. This is shown by the three different arrows for observer 2. Each of the three emission lines shown on the left side of the slit was diffracted a different angle and each has a different wavelength. The diffraction equation discussed in lecture and reproduced below can be used, with the distances from the slit to the observed line and from the slit to the grating to determine the wavelengths of the observed lines of light. Figure 4 Schematic of the Meterstick Spectroscope, seen from above.
Arrows indicate the path of light.
A derivation of the fundamental diffraction equation required for analysis of the spectral data was given in lecture and is reproduced here. In order for constructive interference to occur at the angle , waves from the upper ruling on the diffraction grating must be in phase with waves from the lower ruling, as shown in Figure 5. A beam of light from the discharge tube is collimated by the slit and strikes the diffraction grating. Here m is an integer called the order of diffraction, d is the spacing between centers of adjacent rulings on the diffraction grating, and is the angle, relative to the direction of the incident beam, at which constructive interference occurs. In this experiment, you will observe the first-order diffraction pattern, so that m is always equal to 1.