The light beam is bent, or refracted, as it passes from one material into the next (click to enlarge the diagrams below). In general, when a beam of light (the incident beam) hits the interface between two transparent materials, such as air and water, part of the beam is reflected and part of it continues through the interface and on into the other material. At this angle, called the critical angle, all the light is reflected back into the water. Also notice that the transmitted beam is bent, or refracted.Įxperiment until you find the angle at which the transmitted beam completely disappears. Notice that the beam reflected into the water grows brighter as the beam transmitted into the air becomes dimmer. Slowly change the angle at which the beam of light hits the surface of the water. Notice that most of the beam leaves the water and only a faint beam is reflected back down into the water (click to enlarge the illustration below). You can also search for the beam and track it with a piece of paper.) (Dust in the air helps you see the refracted beam. In the aquarium, you may be able to see both the reflected beam, which bounces back into the water, and the refracted beam, which comes out of the water and into the air. Point the beam so that it hits the surface of the water at just about a right angle. (With the Mini Maglite, you can seal the light in a watertight plastic bag and place the light right in the water!) The beam will be more visible if you can dim the room lights. You can shine the beam into the water through the transparent bottom of the aquarium, or in through a side wall. Direct the light beam upward through the water so that it hits the surface of the water from underneath.

Angle of reflection diagram