To measur the wavelenth of the laser light by using a steel rule as a diffraction grating.
He-Ne Laser & Steel Rule
Specular reflection of the laser beam from a steel ruler gives a zero order image. In Fig.1 the distance h0 and D determine ,the angle of reflection:=arctan(D/h0) and hence also , the angle of diffraction for the nth order is
Let d be the separation of lines on the steel ruler. For the nth order, the path difference between waves from successive ruler lines is =AC-BE. But and .
Put the squared paper on the wall, put the steel rule on te horizontal desk. Open the laser light generator and let the laser irradiate on the separated lines on the steel rulers.
Fig.3 below shows the setup schematicly.
The results are shown below:
d=1/64 inch=0.03969 cm
Wavelength Lambda_1=6.33*E-5 cm =633nm
Wavelength Lambda_1=6.28*E-5 cm =628nm
Wavelength Lambda_1=6.36*E-5 cm =636nm
Now we must pay attention to the measurement of the distance D between the wall and the steel ruler. The fig.4 below tell us that there is not a spot but a strip on the steel ruler when the laser irradiates on it.
So we choose the center of the strip as one terminal of the distance D.