Certain Planes of a Crystal of Halite Have a Spacing $0.399 \mathrm {~nm}$

Multiple Choice

Certain planes of a crystal of halite have a spacing of $0.399 \mathrm {~nm}$ . The crystal is irradiated by a beam of x-rays. First order constructive interference occurs when the beam makes an angle of $20 ^ { \circ }$ with the planes of the crystal surface. What angle does the beam make with the crystal planes for second order constructive?

A) $40 ^ { \circ }$
B) $37 ^ { \circ }$
C) $49 ^ { \circ }$
D) $46 ^ { \circ }$
E) $43 ^ { \circ }$