PURPOSE: Demonstrate the Land effect using red and white light.

DESCRIPTION: This experiment must be done in the dark. Two slides of the same object which were taken in white and red light, are lit by white and red light respectively and superimposed on a screen using slide projectors with zoom lenses and horizontal and vertical adjusters. The intensity of the red light is then adjusted so that the full spectrum of colors is seen in the picture.

This visual illusion, known as the Land effect, is due to the way your eye sees color by comparing adjacent areas for their relative spectral structure, not by viewing light spectra in an absolute way. This is the reason why your eye sees colors basically the same in most kinds of indoor or outdoor lighting, whereas film or video under different lighting conditions produces vastly different coloration.

SUGGESTIONS: This is a really impressive demonstration, if you happen to be covering this area.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for information on this topic and a Scientific American article from which the slides were taken.

EQUIPMENT: Slide projectors with zoom lenses and positioning and intensity adjustments, slides of clown taken in red and white light, red filter.

SETUP TIME: 5 min. (Final adjustment must be made in the class.)

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