1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Interference-thin glass

  1. Sep 7, 2011 #1
    As shown in the diagram , why only ray 2 and 5 are considered? When ray 4 strikes the top interface from underneath, some is reflected. Why this ray is not considered?
  2. jcsd
  3. Sep 7, 2011 #2


    User Avatar

    Of course, you may calculate higher orders (4 reflects, then travels parallel to 3 - call it 3A, then 3A reflects from bottom surface, making 4A ray, which then refracts to 5A).

    Two issues:
    1. every such reflection is much dimmer, than previous ones. So the interference pattern from the first order is the most visible;
    2. phase shift 5-5A is exactly the same, as as 2-5. Constructive interference occurs when 2-5 phase shift is n*2π. For 5A you'll have 2*n*2π - still positive interference. But - to be honest - this argument works only for positive intereference! For the angles exhibiting negative interference, every second reflection acts opposite.
  4. Sep 8, 2011 #3

    Claude Bile

    User Avatar
    Science Advisor

    Secondary reflections are typically quite weak. Standard Fresnel reflections from air/glass for example is about 4-5% in power. The secondary reflections are then at most (0.05)^2 - 0.25% of the original wave. Subsequent reflections rapidly drop off in power.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook