Observing Objects in Water

An object seen in the water will usually appear to be at a different depth than it actually is, due to the refraction of light rays as they travel from the water into the air. This tutorial explores how fish, observed from the bank of a pond or lake, appear to be closer to the surface than they really are.

The interactive tutorial initializes with a human eye observing a fish beneath a layer of water. Light rays reflected from the fish are refracted at the surface of the water, but the eyes and brain trace the light rays back into the water as thought they had not refracted, but traveled away from the fish in a straight line. This effect creates a "virtual" image of the fish that appears at a shallower depth. The Water Depth slider can be employed to increase or decrease the depth and demonstrate changes in the refraction angle and position of the virtual image.

As light passes from one substance into another, it will travel straight through with no change of direction when crossing the boundary between the two substances head-on (perpendicular, or a 90-degree angle of incidence). However, if the light impacts the boundary at any other angle it will be bent or refracted, with the degree of refraction increasing as the beam is progressively inclined at a greater angle with respect to the boundary. As an example, a beam of light striking water vertically will not be refracted, but if the beam enters the water at a slight angle it will be refracted to a very small degree. If the angle of the beam is increased even farther, the light will refract with increasing proportion to the entry angle. Early scientists realized that the ratio between the angle at which the light crosses the media interface and the angle produced after refraction is a very precise characteristic of the material producing the refraction effect.

A number of phenomena that result from light refraction are often observed in everyday life, including the illusion, created by refraction effects, of the actual depth of a fish in shallow water when observed from the bank of a lake or pond. When we peer through the water to observe fish swimming around the pond, they appear to be much closer to the surface than they really are. On the other hand, from the fish's point of view, the world appears distorted and compressed above the water due to virtual images created by refraction of reflected and transmitted light reaching the eyes of the fish. In fact, due to refraction, a fisherman on the bank appears to be farther away from the fish (from the fish's viewpoint) than he or she really is.

Contributing Authors

Matt Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.