PURPOSE: Identify nearsighted and farsighted eyes using laser speckle.
DESCRIPTION: Project the speckled laser pattern on a screen after expanding the beam to about 5 cm diameter. Remove your glasses for the eye test. Watch the speckle pattern as you move your head from side to side. If the speckles move opposite to the direction of your head motion you are nearsighted; if the speckles move in the same direction as your head motion you are farsighted. Put your glasses back on and repeat the experiment to check the adequacy of your lens prescription.
The eye focuses the speckle pattern at its natural focal point. Parallax between the focal plane of the eye lens and the retina cause the apparent motion of the speckle pattern.
REFERENCES: (PIRA unknown.) See Demonstration Reference File for information about laser speckle and details about how this experiment works.
EQUIPMENT: Laser with beam expander.
SETUP TIME: None.
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