Physics
posted by Roby .
Light of wavelength 600 nm illuminates a diffraction grating. The secondorder maximum is at angle 39.1°.
How many lines per millimeter does this grating have?

Strictly speaking, it depends upon the angle of incidence. Assume it is zero degrees (normal).
Then use the grating equation. Let d be the groove spacing.
n*(lambda) = d sin(theta)
In your case,
lamda (the wavelength) = 600*10^9 m
n = 2
sin(theta) = 0.6307
Solve for d in meters.
Lines per mm = 1000/d(meters)