Which of the following is used in photography ?

The correct answer is option 2. Sodium thiosulphate.

Sodium thiosulphate, also known as sodium hyposulphite or "hypo," is a chemical compound that is commonly used in photography. It serves a crucial role in photographic processing, particularly in the development of film and photographic paper.

In photography, after exposing the film or paper to light, an image is formed due to the reaction of light-sensitive silver halide crystals (typically silver bromide or silver chloride) with photons of light. During the development process, the exposed silver halide crystals are reduced to metallic silver, forming the visible image.

Sodium thiosulphate plays a critical role in the development process by serving as a "fixer." It functions to dissolve and remove the unexposed silver halide crystals from the film or paper, preventing further exposure to light and stabilizing the image. This step is essential for preventing the unexposed areas of the film or paper from darkening over time.

The chemical reaction between sodium thiosulphate and silver halide crystals produces soluble silver complexes, which can be easily washed away, leaving behind the developed image. This process is crucial for creating permanent photographic prints.

Overall, sodium thiosulphate is a key component in the photographic development process, helping to fix and stabilize the image formed during exposure to light.