![]() ![]() ![]() If you don't have a feel for what looks too cloudy, look down the length of the tank.if you can't see the other end of the tank, the laser beam won't make it that far either (for the same reason). ![]() A good rule-of-thumb is, if the water looks cloudy (milky) you've added way too much milk and the beam will become attenuated (scattered out of the path) before it reaches the end of the tank (that's bad). Fill the tank to about two-thirds full of warm water and stir in a few drops of milk to make the laser beam visible by scattering. The tank of sugar water must be prepared at least 24 hrs before intended use to allow enough time for all the sugar to completely dissolve. Thus this demo can also be used as a model of long-distance laser beam transmission (periodic focusing and defocusing with gas lenses) and possibly as an analogy to accelerator strong focusing. This effect is only in the vertical direction (direction of the gradient) and not in the horizontal, and is periodic with every bounce. Depending on the gradient that has been established and the angle and height at which the beam enters the tank, the beam may bounce anywhere from once to several times in the length of the tank.Ī closer observation reveals another interesting effect: a defocusing (divergence) of the beam as it arcs downward toward the higher refractive sugar water and a refocusing as it arcs upward. Following the bounce, the beam will arc upward and back down by "total internal refraction" and so on. A laser beam, directed horizontally down the length of the tank, will be refracted downward in an arc and "bounce" by total internal reflection at the plastic-air interface at the outside surface of the bottom of the tank. This establishes a strong gradient in the index of refraction ranging from n=1.50 (84% sugar at the bottom of the tank) to n=1.33 (pure water, a few centimeters off the bottom). Sugar cubes are added to the tank and allowed to dissolve undisturbed. How it works:Ī long, narrow, plastic tank is filled two-thirds full of warm water adulterated with a light-scattering substance. Total internal reflection, total internal refraction, as well as periodic beam defocusing and refocusing are also shown. It can be used as a model of mirage formation (except that the direction of increasing refractive index is in the opposite direction) or even as a representation of the refraction of seismic waves through the Earth's mantle or the refraction of sound waves in the SOFAR channel. In this case the variable refracting medium is a tank of sugar water with a vertical gradient in the concentration of sugar and a HeNe laser provides the light beam.
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