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## Homework Statement

A very thin sheet of plastic (n=1.70) covers one slit of a double-slit apparatus illuminated by 630 nm light. The center point on the screen, instead of being a maximum, is dark. What is the (minimum) thickness of the plastic?

## Homework Equations

Constructive interference d*sinθ=m*λ where m=0,1,2,3,...

Destructive interference d*sinθ=(m+1/2)*λ where m=1,2,3,...

c=λ*f

c=c0/n

λ1*n1=λ2*n2

## The Attempt at a Solution

Obviously this is a case of destructive interference so the sheet of plastic needs to shift the phase of that wave by λ/2. I am unsure of how to determine the thickness of the plastic necessary to make that happen though.