Answer:
31.25 year .
Explanation:
The thickness of layer must change so the destructive interference may be converted into constructive interference . This can happen if thickness is reduced by λ /4 , so that path difference changes by 2 x λ /4 = λ /2 .
This will convert destructive to constructive interference.
change in thickness required = λ /4
= 525 / 4 nm
= 131.25 nm .
year required to wear off
= 131.25 / rate of decay
= 131.25 / 4.2
= 31.25 year .