Homework Help: Two Offset Slits

1. Jan 28, 2008

dopher

I'm having trouble getting started with this problem. I know the interference formulas and how it works, but I'm having trouble determining the phase difference (if B and C were in phase i could do it easily, but they aren't, how do I find out the phase they are initially apart?)
|----------|------------|
|----------B------------|
|----------|------------|
|----------C------------|
|----------|------------|
A----------|------------O
|----------|------------|
|----------|------------|
|<---x1-->|<-----x2--->|

Consider the above setup, not drawn to scale.

Light of wavelength λ= 475 nm is shined at normal incidence to 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.

a) What is the light intensity measured at the point O when both slits B and C are open?

2. Aug 30, 2009

driedupfish

For a, you have to find the path difference between ABO and ACO. This is 3.142851 E-6. Plug this into phi = 2pi (delta) / lambda. This gets you 41.572874. Now you should have everything you need to solve I = 4(I1) cos^2 (phi/2).

B should be the same, because you're increasing both ABO and ACO by the same amount, therefore not changing the path difference.

...this is not from me...