Interference of Light waves HELP

AI Thread Summary
To calculate the thickness of the oil film on the glass plate, the interference conditions for both constructive and destructive interference must be applied. The equations used are 2nt = m(λ)c for constructive interference and 2nt = (m + 1/2)(λ)d for destructive interference. The user attempted to relate the two equations by setting the ratios of wavelengths but found the answer to be incorrect. It's suggested to explore different integer values for m in the destructive interference equation, as m can take on various odd integers, which may yield the correct phase difference. Further trial and error with these values may lead to the accurate thickness calculation.
yoshima
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I've been trying to get the anwser to this question for awhile. Any help is appreciated. :confused:

1. A glass plate (n=1.61) is covered with a thin, uniform layer of oil (n=1.24). A light beam of variable wavelength from air is incident normally on the oil surface. Observation of the reflected beam shows destructive interference at 549 nm and constructive interference at 727 nm. From this information, calculate the thickness of the oil film.

thanks
 
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find the phase difference

Show what you've tried and where you are stuck.

Here's a hint to get you started: Light will reflect off the surface of the oil and the surface of the glass. These two reflections will interfere. What phase difference must they have to interfere constructively? destructively? How does the phase difference depend on the thickness of the oil?
 
the solution that i though would be right

they way I've bee doing it is by using the formula for both destructive and constructive interference.

Constructive inter. --> 2nt=m*(lambda)c
destructive inter. --> 2nt=(m+1/2)*(lambda)d

Because the thickness of the film is the same --> 2nt = 2nt

:. (m+1/2)/(m) = (lambda)c/(lambda)d = 727/549

and from here solve for mc and sub that into the constructive formula 2nt=m*(lambda)c and you get the thickness.. but teh anwser is not right.

Is there another way of solving this problem or am i solving the wrong way.
 
yoshima said:
Constructive inter. --> 2nt=m*(lambda)c
destructive inter. --> 2nt=(m+1/2)*(lambda)d
In your second equation, what you have as "m + 1/2" could also be m + 3/2, m + 5/2, etc. You need to find the value that makes m an integer. Trial and error!
 
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