### Interference

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Messages: 1 - 2 of 2
• #### Message 1.

Posted by U15216371 (U15216371) on Thursday, 10th January 2013

Hi, I got really stuck on this question and was wondering if someone could help me:

A diffraction grating with 300 line/mm is illuminated with light from a laser. A second order maximum is obtained at an angle of 18 degrees to the straight through direction. Find:
a) The spacing of the grating in metres
b) The wavelength of the laser light
c) The angle at which the first order maximum will be observed

I got 1/300 for part A but I got a bit confused during the rest

• #### Message 2

, in reply to message 1.

Posted by BitesizePhysicsTeacher (U15165682) on Thursday, 10th January 2013

Ok, you need to be careful with these as there are some wee tricky bits.....

a) the 300 lines per mm (sometimes called the 'pitch' of the grating) gives a spacing, d, of 1/300th of a *millimetre* -
i.e. d = 0.001 m ÷ 300 = 3.33 x 10^-6 m

b) uses the equation nλ = d sin θ, rearranged as λ = (d sinθ) / n

where n = 2 (second order maximum)
and θ = 18˚

c) uses the same equation as b) rearranged for sinθ, but with n = 1, it simplifies to sinθ = λ/d

Remember this gives the sine of the angle, so you'll need to find the inverse sine (sin^-1) to find the angle.

[Sorry if you were hoping for the full solution, it's probably better for you to be pointed in the right direction and let you work out the rest for yourself]

Hope this has helped, though.

BPT

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