AP physics 2: Phase change in thin film interference

[AAONA]

Homework Statement

Phase change in thin film interference

Relevant Equations

2t=. 5m/wavelength

180° phase change if n2>n1

47. Two thin layers of material with different indices of refraction are coated on a glass plate. The outer first material has n₁ = 1.404, the inner second material has n₂ = 1.531, and the glass has glass = 1.62. If light is incident from air on the first layer, what is the phase change for light that reflects from the glass?

a. 0°

b. 180°

c. 360°

d. 540°

Apparently the answer is 180, but I don't get it. I assumed since it's low n» high» higher > two consecutive 180° phase changes since moving to a denser meduim produces a 180° phase change, so 360° which is equivalent to zero.


What am I missing?

 

[tech99]

The questions asks about the light reflected from the glass, so must traverse the layers in the opposite direction as well.