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November 24, 2014

November 24, 2014

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

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? ?

- Physics please explain -
**drwls**, Wednesday, November 14, 2007 at 10:00amBob 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 11:41amI meant green. Sorry.

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