Reflected Intensity from layer of oil

[unscientific]

Homework Statement

Homework Equations

The Attempt at a Solution

For the second part, I got an answer that is indpendent of the index of oil? That woud mean that the answer would be the same as putting it directly through water..

No clue about the third part.

For the last part,
Not sure how this is related to a single slit where the I ~ sin²(β) / (β)

 

[unscientific]

bumpp

 

[unscientific]

not sure how to even start the last two parts..