Determine the wavelength of unknown light by laser diffraction method

Objective:

To Determine the wavelength of unknown light by laser diffraction method.

Theory:

 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.

Hence, 

If the value of m is known, the wavelength λ of unknown light can be found out.

Procedure:

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.

Experimental Results:

Vernier constant for the horizontal scale of the microscope:

Table for the determination of the wavelength:

Percentage Error:

 

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