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# Diffraction grating watch

1. Morning,

I’m franticly revising for AQA Physics Unit 2 tomorrow afternoon. It was going ok until I came across diffraction grating. I under stand the formula. But when coming to use it I come unstuck of how to interpret d (grating spacing) Here is an example

A Laser beam of wavelength 630 nm is directed normally at a diffraction grating with 300 lines per mm. Calculate … the angle of diffraction on the first order.

So its 690*10^-9 for wavelength into meters however, for the grating spacing how would I work out the correct value for d ?

The book says 1/d, but I always seem to be getting the wrong answer. If someone could please highlight my fault it would be much appreciated.
2. 300 lines per mm does indeed give the spacing by taking the inverse. (1/lines per m)
The usual thing that causes errors is mixing up the units. There are nm, and mm here.
Convert all to metre. 300 lines per mm is 300 x 1000 lines per metre as there are 1000 mm in a metre. (=3 x 10^5)
1 over this then can be used with 690 x 10^-9 m for wavelength.
3. Brilliant. Thanks for the help.

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