Question

A photographic film was wrapped in a thin, light-proof paper packet and a metal key was sandwiched in between the packet and a sample of uranium salt. The setup was locked in a dark cabinet for a whole week. After a week, the film was developed in a dark room and it was found to have been exposed! In addition, an image of the key was formed, as shown in the figure. Explain why the film was exposed and why the key formed an image on the film. (Hint: Think in terms of the penetrating power.) (3 pts.)

Answer 1

Ans:

The metal key was sandwiched in between the packet containing photographic film and uranium sample. Since the setup was locked in a cabinet and kept in the dark room, there will never be interfering of external light. The film was found exposed to light. This interferance comes within the setup due to uranium.

Uranium is weakly radioactive element. It decays by releasing alpha, beta and gamma radiation and all of these have a similar effect to x-rays. Gamma radiation are electromagnetic radiations like x-rays. These radiations penetrate the metal key into the photographic film. Thus the film gets exposed.

Photographic films are made of silver halides. Silver halides dissociate when stimulated by photons, into elemental silver and halogen. This make the film dark and exposed.