Posted by **Mary** on Wednesday, November 14, 2007 at 9:36am.

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? ?

- Physics please explain -
**drwls**, Wednesday, November 14, 2007 at 10:00am
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.

- Physics please explain -
**bobpursley**, Wednesday, November 14, 2007 at 11:41am
I meant green. Sorry.

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