Diffraction of light question - How do I approach this?

[Micky76]

Homework Statement

Question:
Blue light of wavelength 485.6nm from a star is incident normally on a diffraction grating. The light is diffracted into a number of beams, as shown in Fig 5.4.(attached)

The angular separation of the two second-order beams is 45.72 degrees.
Calculate the number of lines per millimetre on the grating.

Homework Equations

The Attempt at a Solution

I genuinely do not know where to start the question. I know the formula for Young's double slit experiment however this does not seem to relate to it at all. Any suggestions for starting the question would be really appreciated.

 

[vela]

Read your textbook. It surely mentions diffraction gratings.

 

[Micky76]

Never seen the formula before. The question is from an old past paper so it might be off the specification. The only thing relating to path difference that I have seen is the Young's modulus experiment. Anyways I had an attempt, is my working correct?

Θ = 45.72/2 = 22.86 degrees
2*485.6*10^-9 = d sin(22.86)
d = 2.50*10^-6 m
d = 2.50*10^-3 mm
N = 1/d = 1/(2.50*10^-3) = 400
Lines per mm = 400

There is no mark scheme for this so if someone could verify my working that would be great.

 

[vela]

Looks good. With a diffraction grating the only difference is the maxima become sharper otherwise it's the same as the two-slit experiment