Light with a wavelength of about 490 nm is made to pass through a diffraction grating. the angle formed between the path of the incident light and the diffracted light is 9.2° and forms a first-order bright band.
what is the number of lines per mm in the diffraction grating?
i really don't know how to do this!

326.8 lines/mm

Explanation:

The formula for the diffraction is:

where we have

d is the grating spacing

is the diffraction angle

n = 1 (because we are reffering to the first-order maximum)

is the light wavelength

Re-arranging the equation, we can calculate the grating spacing d:

This is the distance between the lines in the diffraction grating: therefore, the number of lines per mm will be

think its the third option