optical activity and how not to work with lasers

optical activity is a property of certain materials where they cause rotation of incoming linearly polarized light around the direction of propagation. If the polarization direction rotates clockwise when viewing from the light source, the material is called dextrorotatory. otherwise, it is called levorotatory. A most common example of an optically active material is a sugar solution, which is what we're measuring above. Optical activity happens in chiral substances and sugars are mostly dextrorotatory, which is why sucrose is also known as "dextrose".

moving on to what we're doing above, the set-up is for measuring the rotatory power of a sugar solution. from the right we have the light source (HeNe laser), then the substance to be measured, an analyzer, and a power meter at the end. the rotatory power (theta_rot) is given by the formula,

theta_2 - theta_1 = theta_rot * l * P / 100

the HeNe laser used here already gives off linearly polarized light due to the brewster window incorporated in the manufacturing. we, therefore, only need to measure the following: theta_1 (azimuth angle of the analyzer of diminishing intensity without test solution), theta_2 (azimuth angle of the analyzer of diminishing intensity with test solution), l (length of the "tube"), and P (concentration of the sugar solution). so what i was doing above when yuting took the photo was actually changing the angle of the polarizer until we get a minimum transmission.

and there you have it! i still have to collate my data into a report but that's basically how you measure rotatory power with two polarizers instead of a saccharimeter.

oh, and, HeNe lasers are high-powered lasers which is why laser safety dictates one should wear safety glasses when working with these type of lasers. tsktsk ishi. reflections are not very harmful, though, but it's still advisable to work with goggles all the time in case of accidents.

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