# Two Light Waves Through Plastic

So I'm on my last try for this question, and I could really use some help, I'm completely clueless..

Two waves of light in air, of wavelength λ = 460.0 nm, are initially in phase. They then travel through plastic layers as shown in Figure 35-36, with L1 = 4.00 µm, L2 = 3.50 µm, n1 = 1.20, and n2 = 1.60. (a) What is their phase difference in cycles after they both have emerged from the layers and arrived at the same horizontal position?
_______________cycles
(b) If the waves at that later position are brought together to a single point, what type of interference occurs?

-closer to destructive interference
-cannot tell from the information given
-closer to constructive interference

2. I know that the change in phase is equal to (L/wavelength)(n-1)

3. I plugged in the given values, and got a phase shift of 4.565 radians for n2 and a shift of 1.739 radians for n1. These added to a shift of 6.304 radians. Since only the decimal matters, it's a shift of .304 radians. However, this is way off, since when I divide by 460nm to get the answer in terms of cycles, I get 661.63, which is wayyyy too large. Even then, I'm clueless on how to answer part b too. Can I please get some help?

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Anybody? I really need help with this one..

Anybody?
The slightest help would be amazing

rl.bhat
Homework Helper
Suppose, when the light enters the plastic sheet, the phase is zero.
The wavelength in the medium of n1 is 391.8 nm.
Number of waves in L1 is 4x10^-6m/391.8 nm. = 10.212
That means 10 full cycles plus 0.212 cycle. Hence phase difference is 0.212 cycle.
Now proceed.

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Dante! Its a small world! This is Doug btw. I can't figure this one out for the life of me...