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Refraction of the image of a pencil in a glass of water. As water is poured into the glass, the image of the pencil is displaced from that suggested by the pencil's position. This is due to the refractive power of the water. Light travels at different velocities in different media, and as a light ray passes from one medium to another it appears to bend. As light travels more slowly in water than in air, the image of the pencil below the surface has been 'bent' away from its true position. To the right is a pencil in a glass without water, for comparison.
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