The substances darken upon exposure to light because they are converted to fine particles of metallic silver. Tiny metallic particles absorb rather than reflect light. They also appear black instead of shiny.
This light sensitive property was utilized to produce black and white photography. Silver bromide was the light sensitive compound on film. Areas of the film exposed to light would darken, creating the negative.
Today, the same process is used to create photochromic or Transitions lenses for glasses.Learn more about Atoms & Molecules