Reflections

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.

Theory


Underwater light reflection angle chart
To understand total internal reflection, refer to the diagram above which shows rays of light traveling upwards through the water and crossing the water-air boundary, where some of it is refracted (“bent”), and the rest of it is reflected back into the water. As indicated, there exists a critical angle at which the amount of transmitted light becomes zero, leaving ONLY reflected light. The critical angle is determined by Snell’s Law, which relates it to the ratio of the indices of refraction:

Critical angle formula

where
nair = index of refraction of air (1.00)
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.
 
To understand the implications this, consider the figure below. For an object at depth “D”, the cross-hatched region in the diagram is where the camera needs to be to see a totally reflected image. Since the critical angle is fixed, as the subject’s depth increases, the minimum possible camera-to-subject distance also increases.
Reflection critical angle for underwater

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/Techniques

This 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.

Wide angle reflection undrewater photograph

-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.

fish reflection underwater photograph


-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.

flying fish reflection underwater photography

-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.
underwater reflection photography



-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.