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 is the wavelength of the $x$ -rays?

A) $0.27 \mathrm {~nm}$
B) $0.21 \mathrm {~nm}$
C) $0.24 \mathrm {~nm}$
D) $0.14 \mathrm {~nm}$
E) $0.17 \mathrm {~nm}$