An interesting phenomenon known as “total internal reflection” can be put to good use by underwater photographers. This occurs at distinct, planar boundaries between media with differing indices of refraction- such as at the surface of the ocean, at thermoclines, and haloclines. This article will explain how this phenomenon occurs and how photographers can best use its creative possibilities.
nwater = index of refraction of water (1.33)
The critical angle for a water-air interface is therefore 41 degrees. At all angles smaller than this, the interface acts as a mirror.
The critical angles for thermoclines and haloclines are MUCH smaller than the water-air interface, since the indices of refraction on either side of the interface are usually very similar. Reflections off such boundaries are technically possible, but far too impractical to be useful for underwater photographers.
Tips/TechniquesThis creative technique may take some getting used to. Here are some tips and suggestions for your consideration:
-Calm, flat water is ideal for getting a perfect reflection - waves can multiply, distort, or eliminate reflections altogether.
-Focusing on a point on the surface directly above the object will evenly distribute the depth of field between subject and reflection. However, you will need to decide the principle point of interest, and focus accordingly.
-As always, it is important to properly illuminate the subject. In this case, the intensity of reflected image is directly related to the intensity of light reflected by the subject. The reflected image will never be as bright as the object, since the path length for the reflected light is always longer (or equal to) the camera-to-subject distance.
-Choose subjects that are as near to the surface as possible to both minimize the light path (maximizing light intensity), and to encompass as much of the object and reflection as possible within the available field of focus.
-Water-air interfaces are not only restricted to the surfaces of bodies of water. They also occur at mask faceplates, and even on the surfaces of air bubbles. So be creative, and look for such interfaces as possible opportunities for interesting photographs.