Two waves of light in air travel through a layer

[Any Help]

Homework Statement

Two waves of light in air, of wavelength 550 nm, are initially in phase. They then both travel through a layer of plastic as shown in the figure, with L1 = 4.00 μm, n1 = 1.40, L2 = 3.50 μm , and n2 = 1.60. What is their phase difference after they both have emerged from the layers, when they arrive at the same point?

Homework Equations

N is the numbers of lemdas
N1=L1*n1/λ
N2=L2*n2/λ
N in air = (L1-L2)/λ n(air)=1

The Attempt at a Solution

N2+Nair-N2=0.909 but the correct answer is 5.71?

 

[TSny]

"Phase difference" is an angle (in radians), not the number of λ's.

 

[Any Help]

"Phase difference" is an angle (in radians), not the number of λ's.

What is the name of the difference in wavelengths?

 

[TSny]

What is the name of the difference in wavelengths?

You could also call the difference in wavelengths the "phase difference". So, the answer of .909 wavelengths is a correct answer. However, does the answer of 5.71 include any units?

 

[Any Help]

You could also call the difference in wavelengths the "phase difference". However, does the answer of 5.71 include any units?

no only number

 

[TSny]

no only number

"Bad form" on the part of the answer sheet.