What phenomenon causes a pencil to look broken when placed in water?

The phenomenon that causes a pencil to look broken when it is placed in water is refraction. Refraction occurs when light travels from one medium to another and changes its speed, resulting in a change in direction. When light enters water from air, it slows down and bends towards the normal, which is an imaginary line perpendicular to the surface at the point of entry.

As light passes from the air into the water and back out again, the change in speed and direction makes the submerged portion of the pencil appear to be at a different location than it actually is. This bending of light creates the optical illusion of a break in the pencil at the water's surface. The effect is particularly noticeable because the pencil appears to be at two different angles when viewed from above the water.

Understanding refraction is important because it explains many common visual phenomena, such as the distortion of objects under water and the way lenses focus light. This principle is often applied in various fields, including optics, photography, and even the design of glasses.