Abstract: When a solution is irradiated by light, a part of that light is transmitted, the other part is absorbed and usually changed into heat energy. If the solution is fluorescent then a part of the absorbed light energy is emitted as fluorescence. The absorbed heat energy results in the change of the refractive index of the solution. If a laser beam with Gaussian spatial distribution is used for irradiation then the formation of the so called thermal lens can be observed. The thermal gradient caused by the laser beam can be calculated by measuring the focal length of the thermal lens. The effectiveness of the Moire technique and the thermal lens method were examined. The results show both methods to have about the same sensitivity, but the thermal lens method was found to be much easier to evaluate. Numerous solutions were examined. Some of them were investigated at their anti-Stokes fluorescence region to examine the possibility of cooling down a solution by means of a laser beam. !5
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