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Wed 2 Apr, 2008 08:34 pm
If a thin film of soap hanging in the air (n = 1.37) reflects dominantly red light (690 nm) What is the minimum thickness of the film? and if this film is on a sheet of glass, (n = 1.52), which is the longest wavelength that will now be predominantly reflected?
I got the thickness of the film as 125.9 nm... using d=lamba/4n, since m is 0 order... but I keep getting the second part wrong. Isn't the second part in first first order, m=1?
What did you assume about the angle to obtain the maximum?
/Kayyam
The second part part of the problem has to take into account that there will be a phase shift at both surfaces instead of just the top surface. So instead of d=(m+1/4)lambda/n you should use d=(m+1/2)lambda/n.
When m=0 then, 126=0.5lambda/1.37 and lambda is 345nm which isn't visible but it is still the longest wavelength. In fact none of the answers are visible - they just keep getting smaller as m increases.
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