When I buy a role of undeveloped film, it has a particular weight. After I have taken a picture but before I develop the film, does it weigh more or less? Does it matter what I take a picture of? — CV, Warrenville, IL
I think that a small number of atoms leave the film when it’s exposed to light, so your exposed film probably weighs less than it did when you bought it. That’s because light causes charge transfers within the grains of silver salts, changing silver-halide molecules into silver atoms and halogen atoms, and the halogen atoms probably leave the film or allow other atoms to leave instead. The silver atoms remain in the film, where clusters of three or four of them form the latent image—a cluster triggers the complete conversion of a silver-halide grain into silver during the development process. But the halogen atoms don’t remain in the silver-halide grains. While it’s possible that these halogen atoms are stabilized in the emulsion, so that the emulsion’s weight remains constant, my guess is that they either diffuse out of the film or displace other atoms in the emulsion. Those displaced atoms would then leave the emulsion. Overall, I suspect that atoms leave the film when it’s exposed and that the film becomes ever-so-slightly lighter.
I should point out, however, that the energy absorbed by the film does have a weight and that if the only effect of exposing film to light were that the film absorbed this additional energy, then the film’s weight would increase by a fantastically small amount. But the chemistry that results from this energy absorption certainly swamps the weight of the light energy.