A beam of X-rays is constructively scattered in second order from the surface of the crystal at an angle of 30° and the spacing between layers of atoms in NACl crystal is 4.5 × 10-10 m. The wavelength of X-rays is ______
1
2.25 × 10-10 m
2
1.25 × 10-10 m
3
2.50 × 10-10 m
4
1.50 × 10-10 m