Question

Discuss the experiment to determine the wavelength of monochromatic light using a diffraction grating.

Answer 1

1. The wavelength of a spectral line can be very accurately determined with the help of a diffraction grating and a spectrometer.
2. Initially, all the preliminary adjustments of the spectrometer are made.
3. The slit of the collimator is illuminated by a monochromatic light, whose wavelength is to be determined.
4. The telescope is brought in line with a collimator to view the image of the slit.
5. The given plane transmission grating is then mounted on the prism table with its plane perpendicular to the incident beam of light coming from the collimator
6. The telescope is turned to one side until the first-order diffraction image of the slit coincides with the vertical cross wire of the eyepiece.
7. The reading of the position of the telescope is noted.
8. 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 the two positions gives 2θ.
9. Half of its value gives θ, the diffraction angle for the first-order maximum. The wavelength of light
   λ = `(sin θ)/"Nm"`
10. Here, N is the number of rulings per metre in the grating and m is the order of the diffraction image.
    
    Determination of wavelength using grating and spectrometer