alternative methods

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alternative methods

Postby Blessy » Fri Nov 16, 2012 9:36 am

can I use this method or a similar method to measure the rate of absorption and transmission of Infrared radiation in various common solutions ?

[Moderator note: question is about this project: Measuring the Sugar Content on a Liquid with a Laser Pointer -- ... p028.shtml]
Posts: 1
Joined: Sat Nov 10, 2012 1:43 am
Occupation: Student:College
Project Question: by passing light through a solution in a hollow prism ( just as the method to find the concentration of sugar in a solution) can I found the absorption and transmission of Infra red radiation of various liquids by analyzing the intensity of transmitted light by any method ?
Project Due Date: 16/11/2012
Project Status: I am just starting

Re: alternative methods

Postby theborg » Fri Nov 16, 2012 6:21 pm


Thanks for your question, and welcome to Science Buddies. What I think you are asking is determining the Phase Velocity of the electromagnetic wave propagation through a medium other than a vacuum. We all know that the speed of light in a vacuum is ~ 3x10^8 m/s (denoted by the symbol, c). That same wave (say a light beam) moving through matter moves at a speed less than c due to the wave being absorbed and reemitted by the atoms of the matter over and over again, like small speed bumps. This slowing of the beam as it transitions from one medium to another causes the beam to bend in a measurable way. This science buddies experiment details how the speed of light in a medium can be measured: ... p009.shtml

To apply it to your experiment, you will need to find a way to accurately detect and measure an invisible infrared beam. However, once all the angles are measured, you can use Snell's law to determine the index of refraction (n2) of your solutions. From there, the relationship n2=c/v2; where n2=index of refraction, c=speed of light in a vacuum, v2=phase velocity through your solution. Rearranging, we get v2=c/n2 to solve for the phase velocity. The difference between c and v2 is the absorption and transmission propagation rate of the wave through the solution. You can further use the relationship v2=c/n2=lambda/T; where lambda=wavelength, T=time to determine the time it takes to propagate.

As far as the intensity of the beam as it enters/exits the material goes, this doesn't account for the reflection properties of the medium. To take it the next step, you can use Fresnel equations of reflection to solve for the reflection (Rs/p) and transmission (Ts/p) coefficients to identify the fraction of the beam that is reflected away vs. what is absorbed vs. what fraction makes it through the medium to be detected on the other side. Basically:

E_in = E_out + E_attenuated

This doesn't take into account scattering, but would solve for the total attenuation within the material itself.

E_attenuated = E_scattered + E_absorbed

I haven't worked with Maxwell's equations in quite awhile, so if anyone has anything to add, please do. I hope this answers the question.
Last edited by theborg on Sat Nov 17, 2012 1:31 pm, edited 1 time in total.
Hope this helps.

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Joined: Mon Oct 03, 2011 12:26 pm
Occupation: US Air Force Space & Missile Operations
Project Question: "To explain all nature is too difficult a task for any one man. 'Tis much better to do a little with certainty and leave the rest for others that come after you, than to explain all things by conjecture without making sure of anything." - Sir Isaac Newton
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Re: alternative methods

Postby John Dreher » Sat Nov 17, 2012 12:40 pm

Blessy wrote:can I use this method or a similar method to measure the rate of absorption and transmission of Infrared radiation in various common solutions ?

Did the previous post answer your question, or are you really interested in infrared absorption in fluids, not velocity of propagation?
John Dreher
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Joined: Sun Dec 25, 2011 8:33 am
Occupation: Astronomer, Professor of Physics, SETI Researcher (retired)
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