All GRE Subject Test: Physics Resources
Example Question #3 : Electromagnetics, Waves, And Optics
A beam of light, with wavelength , is normally incident on a transmission diffraction grating. With respect to the incident beam, the first order diffraction maximum occurs at an angle of . What is the number of slits per centimeter on the grating?
The equation describing maxima of a diffraction grating is:
Where d is the separation of slits, which can also be expressed as:
Substituting d into the first equation and making the small angle approximation, one can solve for (lines per length):
Note that the angle had to be converted from degrees to radians. Finally, the question asks for lines per centimeter, not meter, so the answer becomes .