# A question on CD diffraction

1. Oct 23, 2011

### unscientific

1. The problem statement, all variables and given/known data
White light was shone vertically down on a cd tilted 5°, with adjacent tracks d = 720 nm. An observer some distance away observes a color of light. Determine that wavelength of light observed. (Refer to picture attachment)

2. Relevant equations
finding path difference

3. The attempt at a solution

I understand that finding the path difference is key to solving this problem. So, i found the path difference to be simply:

d cos (90+ 5 - ∅) = (1)(λ)

But the answer wrote path difference =
d cos (90 + 5 - ∅) - d cos (90 - 5)

Using the provided answer, it didnt make sense to me for the additional term of ' d cos (90 - 5)' to be there?

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• ###### cd diffraction2.jpg
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2. Oct 23, 2011

### Spinnor

I think there will be path length differences from the 5 degree tilt of the cd and from the angle of the viewer, see sketch below,

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• ###### block090.jpg
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3. Oct 24, 2011

### unscientific

hmm i still don't seem to be able to grasp the answer..

4. Oct 24, 2011

### Spinnor

Look at the sketch and follow the two paths. Calculate the path length difference from the first dashed line to the second dashed line. I think you will find the required answer.

5. Oct 24, 2011

### unscientific

OH i get it! the tilting causes path difference before and after diffration at surface!

the right beam has to go down more while the left beam has to travel more!