Constructive wave interference problem

Homework Statement

A sheet of glass is coated with a 505nm thick layer of oil (n = 1.42).

For what visible wavelengths of light do the reflected waves interfere constructively/destructively?

Homework Equations

For constructive interference

lamba=2nd/m

For destructive interference

lamba=2nd/(m-1/2)

The Attempt at a Solution

for constructive

2*1.42*505/(3 or 4) (since m=3 and 4 will give me a visible wavelength) lamba = 717 nm, 478 nm

for destructive

2*1.142*505/(3 or 4 + 1/2) (since m=3 and 4 will also give me a visible wavelength) lamba = 574, 410 nm

HOWEVER, they are wrong... so im completely clueless of what to do next.. help please >.<

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Doc Al
Mentor

A sheet of glass is coated with a 505nm thick layer of oil (n = 1.42).
Hint: How does the index of refraction of the oil compare to that of the glass. Does it matter?

glass is supposed to have a higher index of refraction.. yes it matters...

Doc Al
Mentor

Hmmm... in that case your solution looks fine to me.