In a real CD, the pits shown in the figure below are covered with a layer of pla

Question

In a real CD, the pits shown in the figure below are covered with a layer of plastic so that surface scratches do not spoil the pits.

If the index of refraction of the plastic coating is n=2.45, what minimum depth should a pit have to give destructive interference using light with lamdba = 630nm????????

Please also let me know under which condition m =0, m=1 ,m=2

In a real CD, the pits shown in the figure below are covered with a layer of plastic so that surface scratches do not spoil the pits. If the index of refraction of the plastic coating is n=2.45, what minimum depth should a pit have to give destructive interference using light with lamdba = 630nm???????? Please also let me know under which condition m =0, m=1 ,m=2

Explanation / Answer

Because 2 rays will be reflected, you use the destructive interference equation for 2 phase reversals:

2nt=(m+0.5)(wavelength) where t is thickness
set m=1
t=wavelength / (4n)
t= (630 nm) / ((4)(2.45))
t= 64.28 nm thick

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