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?

Question

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


For Further Reading

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.