What phenomenon occurs when light from a flash unit strikes suspended particles in water at a straight-on angle?

Backscatter occurs when light emitted from a flash unit strikes suspended particles in water at a straight-on angle. This phenomenon happens when the light bounces off these particles and travels back toward the camera or the observer instead of illuminating the subject. This results in a bright, often unflattering glare in underwater photography, as the light reflecting off the particles can reduce the visibility of the actual subject and create a washed-out image.

In contrast, reflection refers to the bouncing of light off a surface but does not specifically relate to the interaction with particles in the water. Diffusion involves the scattering of light as it passes through a medium, which is a different process. Refraction is the bending of light as it passes from one medium to another, altering its speed and direction based on the properties of the media involved. While all these processes deal with light, only backscatter directly pertains to the situation where light strikes particles in water at a straight-on angle.