The phenomenon that causes a pencil to appear bent or broken when placed in a glass partially filled with water is called refraction. Refraction occurs when light travels from one medium to another (in this case, from air to water), which changes the speed of light and causes a change in its direction.
When light rays pass from air (a less dense medium) into water (a denser medium), they bend toward the normal (an imaginary line perpendicular to the surface at the point of incidence). As a result, the part of the pencil that is underwater appears to be at a different position than the part that is above the water, creating the optical illusion of the pencil being bent or broken at the water's surface.
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