Question

Can you please explain your answer. How did you get blue lamda. Is it because red and green makes blue? How do I find the value of blue lambda?


A mixture of red light (vacuum = 661 nm) and green light (vacuum = 551 nm) shines perpendicularly on a soap film (n = 1.35) that has air on either side. What is the minimum nonzero thickness of the film, so that destructive interference causes it to look red in reflected light?
nm



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Physic please help! - bobpursley, Wednesday, November 14, 2007 at 7:30am
Wouldnt the thickness be blue lamda/4? ?

Answer 1

Bob may have meant to say green light, not blue. The wavelength requirement for minimum reflection is given at
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/soapfilm.html
For normal incidence and first order destructive interference of reflected green light,
2 N d = lambda(green)
N is the index of refraction.

Answer 2

I meant green. Sorry.