Refraction of the image of pencils in glasses of water. The image of the pencil in each glass of water has been 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.