In my collection of computer disks are several that I cannot tell if they are CD

Question

In my collection of computer disks are several that I cannot tell if they are CD regular DVD or Blu-ray DVD discs. It was suggested I can use my laser pointer to determine which is which.Since the discs consists of closely spaced metallized grooves a multi slit diffraction pattern characteristics of the groove space and should be created. The amount of information stored increases going from CDs to DVDs to Blu-ray DVD and hence I expect the groove spacing it to be largest on CD and smallest on the Bluray with the regular DVD somewhere between.

Explanation / Answer

The condition of diffraction grating is              m = d sin      here m = 1, d = 1.6x10^-6 m and = 19.5 , so           = (1.6x10^-6s) sin 19.5              = 5.34x10^-7 m              = 534x10^-9m              = 534nm

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