When a pencil is placed partly in a glass of water, it appears bent or broken at the water's surface due to an optical phenomenon called refraction.

Here's how it works:

1. **Light travels at different speeds**: Light travels faster in air than in water. When light moves from one medium to another (in this case, from air into water), its speed changes.

2. **Change in direction**: As light rays pass from air into water, they bend or change direction. This bending occurs because the light rays slow down when they enter the denser medium (water).

3. **Visual perception**: Your eyes perceive the pencil as being in a straight line, but because the light rays are bent, your brain interprets the position incorrectly. The part of the pencil that is underwater seems to be in a different place than the part above water, making the pencil appear bent where it enters the water.

This effect is not only observed with pencils but also with any object placed at the boundary between two different mediums, such as air and water. It’s a common demonstration used to illustrate the principles of refraction in physics.