PURPOSE: To demonstrate focusing by the parabolic surface of a rotating liquid.
DESCRIPTION: A container of glycerine darkened by blue food coloring is rotated at 45 RPM so that it forms a parabolic mirror with a focal length of about 22 cm. A lighted object 70 cm above the surface forms a virtual erect image 70 cm below the surface when the liquid is quiescent, and a real, inverted image 32 cm above the surface when the liquid is rotating. A ruler is positioned at the height of the real image for reference.
The image can be viewed directly individually from above or it can be shown to the entire class using the TV camera and monitor. The difference in the focus for the two images is seen very nicely; the real image is clearly in focus with the ruler, while the virtual image is both inverted and out of focus.
Many modern telescope mirrors up to 25 feet in diameter are constructed using this technique of "spin-casting." A container of molton glass is rotated as it cools, forming a nearly perfect parabolic mirror, the surface of which is then ground to eliminate small errors and coated with a reflecting material.
REFERENCES: (PIRA 6A20.42) See Demonstration Reference File for AJP article describing this apparatus and information about its application to spin-casting of telescope mirrors.
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