Final answer:
Cracks in a bent film emulsion appear as white lines due to the absence of metallic silver in the cracked areas, which should have developed an image.
Step-by-step explanation:
Cracks in the film emulsion caused by excessive bending of a film will appear as white lines. This happens because when the film is bent too much, the emulsion layer can crack, leading to areas where the silver, which develops the image, is absent. As a result, those areas will not develop properly and instead reflect back as white lines. It is important to understand that these are not clear areas; they reflect the color white because there is a lack of metallic silver in those areas that could create an image.
The integrity of the thin film is compromised, which interrupts the otherwise uniform development of the images created by silver reduced from radiation-sensitive silver compounds in the film's emulsion.