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Polarization in sugar solution
188
Polarimeter
Polarization in Sugar Solution
By: Ryan, Darren, Brandon,
Calder, Vincent
Light is a type of electromagnetic wave. Electromagnetic waves are transverse waves consisting of varying electric and magnetic fields that oscillate perpendicular to the direction of propagation. There are specific directions for the oscillations of the electric and magnetic fields. Polarization is the attribute that a wave’s oscillations have a definite direction relative to the direction of propagation of the wave. Waves having such a direction are said to be polarized. For an EM wave, we define the direction of polarization to be the direction parallel to the electric field.
Polarization
There are materials which rotate the plane of polarization of the light that goes through them. These substances are called optically active. Among these substances is sugar. Sugar molecules are chiral, and they 'rotate' the polarization of light that passes through a sugar solution to a slightly different polarization. Measurement of this change in polarization orientation is called polarimetry. This is actually one of the ways sugar concentrations are measured in industry.
Polarizers
Polarimeter
A polarimeter is a type of instrument for measuring the optical rotation of a substance. By measuring the optical rotation, the polarimeter can be used in analyzing the concentration, content, and purity of a substance.
P is the polarizer for producing linear polarized light. A is the rotating polarizer (the analyzer) for measuring the rotary angle combined with the angle scale. Normally, the adjustment of total darkness are at crossed polarizers (the transmission axes of polarizer and analyzer are perpendicular to each other) When an optically active substance(the sugar solution) is present in the beam, it rotates the polarization of the light reaching the analyzer, so that there is a component that reaches the detector. The angle at which the analyzer must be rotated to return to the minimum detector signal is the optical rotation
Sugar
If you shine polarized light through a sugar solution, the sugar will rotate its polarization to a different angle. How far it gets rotated depends on:
a) The type of sugar (which is different for different sugar molecules)
b) What thickness of sugar solution it passes through
c) The concentration of the sugar solution.
You could measure the rotation with polarizers and deduce the concentration of the sugar.
Start by placing one polarizer at a fixed angle. This polarizer won't move at all during the experiment. Then place the second polarizer in front of the first one, and shine some light through them both. You could use a flashlight or the light from a sunny window as the light source. Rotate the second polarizer—you should see the intensity of the light change. Find the angle of the second polarizer that blocks as much as the light as possible. Mark this angle, maybe by drawing a vertical line on the polarizer. The two polarizers are now oriented perpendicular to each other, so light that makes it through the first polarizer won't make it through the second one at all.
Now put the sugar solution in between the two polarizers. The amount of light that makes it through the second polarizer should change, because the polarization was rotated by the sugar. Rotate the second polarizer until the amount of light that gets through is minimized again. Measure how far you had to rotate it from its original position. This angle is the rotation of the light caused by the sugar solution.
To calculate the concentration of sugar, you can use this formula:
The rotation of the polarization will depend on the wavelength (color) of the light. White light is a mixture of all colors, and the different colors will be rotated by different amounts. You could try putting some kind of color filter before the first polarizer to narrow the range of colors, or you could use an inexpensive laser pointer as your light source instead of white light. You should also try to find a specific rotation value that was measured near the wavelength you're using. A common measurement wavelength is 589 nm, which is emitted by the sodium-vapor bulbs that are commonly used in streetlights.
Links:
https://van.physics.illinois.edu/qa/listing.php?id=42894&t=measuring-the-concentration-of-sugar-with-polarized-light
http://zdn.umed.lodz.pl/~biophysics/Module_II_lab26.pdf
https://courses.lumenlearning.com/physics/chapter/27-8-polarization/