Explanation:
When a black and white photographic film is exposed to light, the grey color on the film is due to the presence of silver (Ag).

Formation of Image on Photographic Film:
1. The photographic film consists of a layer of gelatin that contains tiny silver bromide (AgBr) crystals.
2. When light falls on the film, it interacts with the silver bromide crystals.
3. The light energy causes the silver bromide crystals to get activated and undergo a chemical reaction.
4. The activated silver bromide crystals break down into silver (Ag) and bromine (Br2) ions.
5. The silver ions combine with the electrons present in the gelatin layer of the film to form metallic silver (Ag).
6. The metallic silver particles form clusters, which are responsible for the grey color on the film.

Role of Silver:
Silver plays a crucial role in the formation of the image on the photographic film. The silver ions produced during the reaction with light interact with the gelatin layer and form metallic silver. These silver clusters are responsible for the grey color observed on the film.

Role of Bromine:
Bromine (Br2) is also involved in the reaction, but it does not contribute directly to the grey color on the film. Bromine ions are released when the silver bromide crystals are activated by light. However, it is the silver ions that interact with the gelatin layer and form metallic silver, resulting in the grey color.

Conclusion:
In conclusion, the grey color observed on a black and white photographic film when exposed to light is due to the presence of silver (Ag). The silver ions produced during the reaction with light combine with the gelatin layer to form metallic silver clusters, which give the film its characteristic grey color. Bromine (Br2) is also involved in the reaction as it is released when the silver bromide crystals are activated, but it does not directly contribute to the grey color on the film.