diffraction grating?

Question

A diffraction grating with 230 lines per mm is used in an experiment to study the visible spectrum of a gas discharge tube. At what angle from the beam axis will the first order peak occur if the tube emits light with wavelength of 686.2 nm?

At what angle will the second order peak occur?

i used the sin theta = wavelength/distance eqn and i got .00904 for the first one and .0181 for the second one and i got both of them wrong any suggestions?

Answer 1

I get arcsin(686.2E-9/(0.001/230)) = 0.1585 for first, arcsin(2*686.2E-9/(0.001/230)) = 0.3211 for second.

Answer 2

Beam Axis