To Determine the wavelength of unknown light by laser diffraction method.
If a parallel beam of light of wavelength λ is coming out from a diffraction grating, placed vertically on the optical bench, then the diffracted rays from the grating will form at the focal plane, a number of primary maxima of different order numbers(n) on both sides of the central maxima of zero order. If θ be the angle of diffraction of nth order primary maxima then sinθ=mnλ where m is the number of rulings per cm of the grating surface.
If the value of m is known, the wavelength λ of unknown light can be found out.
Measuring the wavelength of the laser
- Set up the laser and grating as shown in the figure.
- Measure the distance L between the meter stick and the diffraction grating.
- Turn on the laser and center the meter stick at the 0th order image.
- Measure the distance s1 between the 1st order images appearing on the left and right sides of the center line.
- Measure the distance s2 between the 2nd order images appearing on the left and right sides of the center line.
It should be clear from simple trigonometric considerations that
from which you can extract two independent measurements of the laser wavelength.
Vernier constant for the horizontal scale of the microscope:
Table for the determination of the wavelength: