Why does a pencil bend in water?

The pencil is still straight. The light carrying its image changes direction as it crosses the boundary between water and air.

Light changes speed

Light travels at different speeds in different materials. When light crosses a boundary at an angle, this change in speed usually changes its direction. We call this refraction.

Your brain interprets incoming light as if it travelled along a straight path. Light from the underwater part of the pencil reaches your eye along a refracted path, so that part appears shifted. The part in the air appears in a different position, creating the apparent bend.

Investigate it

1. Put a pencil in a clear glass of water and look from the side.
2. Change your viewing angle. Does the apparent bend change?
3. Compare a pencil in an empty glass with one in a full glass.

The curved glass also refracts light. For a simpler comparison, use a clear container with flat sides.

Make the connection

Refraction helps explain lenses, glasses and why a swimming pool can appear shallower than it really is.

Illustration or photograph provides context; use the explanation to understand the science.