Oil layer thickness for constructive interference at 590nm

[MyNameIsKhan]

Right, so, I am new to this, so please excuse me if I make a glaring error.

Say there is can of water with a layer on oil on the surface. Light (of lambda=590nm) is shone straight down on the oil. How do I find out the thickness of the oil layer that would most strongly reflect the light?

 

[sophiecentaur]

Hi and welcome to Physics Forums
Is this a homework question, by any chance? You are referring to thin film interference, I guess. What is the condition for the waves, reflected from both surfaces, to be in phase when they emerge from the surface? PF doesn't usually just give the answers to question like this one and what I have written should help you to find the right answer. (Or you could look at the Hyperphysics website if you don't have a textbook)