UPSC NDA 2 2021 General Ability Solved Paper

Concept:Silver bromide ($\mathrm{AgBr}$) is a light-sensitive compound, which makes it ideal for traditional black and white photography.Explanation:Black and white photographic film is coated with a layer containing silver bromide crystals.When light falls on the film during exposure, the silver bromide decomposes according to the reaction:$$2\mathrm{AgBr} + h\nu \text{ (light)} \rightarrow 2\mathrm{Ag(s)} + \mathrm{Br}_2$$This reaction forms tiny specks of metallic silver ($\mathrm{Ag}$) on the film.The amount of silver produced depends on the intensity of light at each point of the image.The exposed film is then developed using a chemical process that converts the silver specks into a visible image.The unexposed areas, where no silver formed, are washed away, leaving the final black-and-white photograph.Silver bromide is chosen because it is much more sensitive to light than other silver halides like silver fluoride ($\mathrm{AgF}$).The options $\mathrm{AgCO}_2$ and $\mathrm{Ag}_2\mathrm{SO}_4$ are not commonly used in photography.Answer:The correct compound is silver bromide, $\mathrm{AgBr}$. Therefore, option B is correct.