I am trying to determine the total number of primary maxima that can be observed when light of wavelength 500 nm is incident normally on a diffraction grating, with the third-order maximum of the diffraction pattern observed at 32.0 degrees.
Rearranging the diffraction grating formula for maxima number ( $m$ ):
$$ m= \frac{d \space \sin \space\theta_\text{bright}}{\lambda} \, . $$
I can get the right answer if I let $$\theta = 90 ^\circ \, .$$ However, I do not understand why this angle value is used.
Best Answer
any angle over theta 90 will mean that the diffraction will be going behind the diffraction gratings which is impossible. so 90 is the maximum that you can get this is why you have to round down the decimal answer you will get.