Consider the above setup, not drawn to scale.
Light of wavelength λ= 475 nm strikes the first screen with slit A.
The second screen, x1 = 0.7 meters behind the first screen, has two slits, B and C .
The third screen is x2 = 1.5 meters behind the second screen. It has slit O, which is level with slit A. A lightmeter measures the light intensity at the slit O.
When light is sent through slit A and measured at the slit O with either slit B or slit C open one slit at a time, the intensity at the point O is the same: I0 = 0.5 W/m2. (The slit widths can always be adjusted so that this is true, but for this problem you can/should ignore the width of all slits.)
Slit B is at height y1 = 2 mm above slit A.
Slit C is at height y2 = 1 mm above slit A.
Note that the drawing is not drawn to scale.
1) What is the light intensity measured at the point O when both slits B and C are open?
Law of cosine
The Attempt at a Solution
I believe that the light enters B and C is out of phase. I tried to find delta using the Pythagorean theorem and got 3.14e-6. Then I used 2*pi*delta/lamba to get what i thought would be phi. Then i did 4*.5*cos^2(phi/2)...but that didn't work. I got something around .256 but that was incorrect