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## Homework Statement

a narrow beam of laser light (i.e coherent monochromatic light) wavelength 630nm is incident on a grating having 300 lines per mm. a piece of paper is curved 180 degrees beyond the grating. calculate how many spots of red light should be seen

## Homework Equations

n[itex]\lambda[/itex]=d*sin[itex]\theta[/itex]

d = spacing between lines

n = order of angle

## The Attempt at a Solution

so theres 300 lines per mm and i want to find out d therefore i did [itex]\frac{1}{300000}[/itex] and got 3.33x10^-6 lines per m

so now i can use n*630x10^-9=3.33x10^-6*sin(90)

this makes

n=[itex]\frac{3.33x10^-6*sin(90)}{630x10^-9}[/itex]

this gives me 4.72 which I can round up to 5 and multiply by 2 to get the order for 180 degrees.

so I get an answer of 10 and the books answer is 11.

what am I doing wrong

P.S first time using latex...