QS Study

Experiment to determine the wavelength of monochromatic light using a plane transmission grating

The wavelength of a spectral line can be very accurately determined with the help of a diffraction grating and spectrometer.

Initially, all the preliminary adjustments of the spectrometer are made. The slit of the collimator is illuminated by a monochromatic light, whose wavelength is to be determined. The telescope is brought in line with the collimator to view the direct image. The given plane transmission grating is then mounted on the prism table with its plane is perpendicular to the incident beam of light coming from the collimator. The telescope is slowly turned to one side until the first order diffraction image coincides with the vertical cross wire of the eyepiece. The reading of the position of the telescope is noted (Figure).

a2Fig: Diffraction of monochromatic light

Similarly, the first order diffraction image on the other side is made to coincide with the vertical cross wire and corresponding reading is noted. The difference between two positions gives 2θ. Half of its value gives θ, the diffraction angle for the first-order maximum. The wavelength of light is calculated from the equation λ = sinθ/Nm. Here N is the number of rulings per metre in the grating.