Two Beams of Light Phase Difference

[dwk17]

Homework Statement

A beam of light of wavelength 606 nm passes through two slabs of material of identical thickness d= 1.40 micrometers, as shown in the figure. The slabs have different indices of refraction: n1= 1.51 and n2= 1.68. What is the phase difference (in radians, do not enter units) between the two parts of the beam after it passes through the slabs?

Homework Equations

wavelength(vacuum)/wavelength = n

The Attempt at a Solution

I calculated the wavelength inside both materials using the equation above (4.01*10^-7m & 3.61*10^-7m). Then i divided the distance of the slab by the wavelength for each (3.41 & 3.89). I found the difference between these two numbers (0.31) which is the wave difference, how do I get phase difference from this?

 

[mfb]

A difference of 1 wavelength corresponds to a phase of 2 pi (1 revolution in a circle).