# Illustration Bragg reflection on two lattice planes

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An incident X-ray wave (shown here as a beam) of wavelength $$\class{violet}{\lambda}$$, is reflected at two adjacent lattice planes (dots are lattice atoms). The glancing angle $$\theta$$ is the angle at which an interference maximum occurs in the interference pattern.

The X-ray wave reflected at the deeper lattice plane has to travel a larger path. This path difference corresponds to exactly TWICE: $$\class{blue}{d} \,\sin(\theta)$$. However, the path difference must also equal a multiple of the wavelength for constructive interference of the two waves to occur: $$m \,\class{violet}{\lambda}$$.