Some watch dials are coated with a phosphor, like ZnS, and a polymer in which some of the 1 H atoms have been replaced by 3 H atoms, tritium. The phosphor emits light when struck by the beta particle...

Some watch dials are coated with a phosphor, like ZnS, and a polymer in which some of the¹H atoms have been replaced by³H atoms, tritium. The phosphor emits light when struck by the beta particle from the tritium decay, causing the dials to glow in the dark. The half-life of tritium is 12.3 yr. If the light given off is assumed to be directly proportional to the amount of tritium, by how much (in percentage) will a dial be dimmed in a watch that is 50 years old?