Final answer:
Mottled density is caused by thin film interference, where light reflected from a film's layers create constructive or destructive interference patterns. This effect can be observed in phenomena such as soap bubbles and is central to holography.
Step-by-step explanation:
The mottled density described in the question appears to be related to the photographic or film process and is most likely referring to the effect of thin film interference which occurs when light waves reflected from the top and bottom layers of a thin film, such as a soap bubble or film emulsion, interfere with each other. This can cause some wavelengths of light to be amplified (constructive interference) and others to cancel out (destructive interference), leading to the appearance of a mottled or foggy density on the film. Another term related to this phenomenon is holography, which differs from traditional photography by capturing the wave characteristics of light rather than using geometric optics. The recording of interference patterns is integral to understanding holography and involves using constructive and destructive interference to create an image on a film without the use of lenses.