Grating equation
= Grating equation
{title2=$m\lambda=d(\sin\alpha+\sin\beta)$}
For a reflection <diffraction grating> with both ray directions measured on the same side of its normal, adjacent grooves have path difference $d(\sin\alpha+\sin\beta)$. <Constructive interference> requires this to be an integer multiple $m\lambda$. Signed-angle conventions can turn the sum into a difference; the physical path difference is unchanged.