Wednesday, December 30, 2009

Optical Absorption

For one of the classes that I teach the students use a device called a spectrophotometer. Basically this device works by shining a light through a sample and measuring the amount of light that gets through the material. This device is more than a fancy light meter which simply measures intensity. This device can break down the transmitted light into specific wavelengths and thus measure the amount of light transmitted for each wavelength. By measuring the amount of light transmitted we can figure out the the absorption which can tell us something about the specific properties of the material.

In this case my students are trying to determine the band gap energy for different semiconductors and also whether or not the band gaps are direct or indirect. I plan on posting about this lab at some point, but for now I just wanted to mention one thing that I measured. While I was in the lab one day I put my glasses in the detector and measured the absorption of the plastic lenses. The results I got are shown in the graph below (click on it to view it bigger).
The wavelengths that I measured extended from 800 nm (infrared) to 200 nm (ultraviolet). In the graph I only show down to 400 nm because below that my glasses absorbed too much light and the detector could not get accurate readings. Still you can see a sharp increase in the the absorption as you approach 400 nm. Other than the region below 400 nm my glasses did not absorb much light. This can be seen by the low measurements that are all below 0.05. To give you an idea of what this means, all throughout the visible spectrum ( ~400-720 nm, except for the tiny bit close to 400 nm) my glasses absorbed less than 10% of the light. An absorption rating of 1 would mean that an object absorbs 90% of the light, a rating of 2 would mean it absorbs 99%, 3 99.9% and so on. Typically an absorption rating of 7 (99.99999% absorbed, yes that can be measured) or higher is considered fully opaque.

As you might notice the absorption of the plastic in my glasses is not constant. There are small bumps or waves in the graph. These bumps are a result of the the thickness of my glasses and some slight internal reflections inside the material. These bumps are useful because from the size and shape of the bumps we can figure out the index of refraction for the material that my glasses are made from. That would be useful information to know if I were the one designing the glasses since the index of refraction determines the corrective ability of the lenses, and the curvature needed to make them work. Unfortunately I did not put my glasses in straight in the detector which meant that the path of the beam was not normal (perpendicular) to the surface of the lens. This makes the measurement more difficult (i.e. it would take more work than I'm willing to put into it) so I was not able to figure out the index of refraction for my glasses.

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