Triple Slit optics interferece

[tboyers]

Homework Statement

A triple-slit experiment consists of three narrow slits, equally spaced by distance d and illuminated by light of wavelength λ. Each slit alone produces intensity I1 on the viewing screen at distance L.
Consider a point on the distant viewing screen such that the path-length difference between any two adjacent slits is λ.

What is the intensity at this point?
Express your answer in terms of I1

What is the intensity at a point where the path-length difference between any two adjacent slits is λ/2?
Express your answer in terms of I1.

Homework Equations

I have no idea what applies to this.

The Attempt at a Solution

I have no idea how to solve this. I know that the answers are 9I for the first question and I for the second. I found it in a list of problems to know for my final exam, but i have no clue how to start.

 

[TSny]

Consider just two slits for the moment. If the path difference for the waves from the two slits is λ, do the waves arrive at the screen in phase or out of phase?