# Diffraction of light question - How do I approach this?

## 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.

## 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.

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vela
Staff Emeritus
Homework Helper

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
Staff Emeritus