NDA (II) 2021 General Ability Test · Question 102 of 149

Correct — B and C, silver bromide (AgBr) and silver chloride (AgCl). UPSC's official final key accepted BOTH options as correct, so full marks were given for either. Both are silver halides that darken when light strikes them: light energy breaks the compound down and deposits fine grains of black metallic silver, which is exactly the light-sensitive change that records an image on black-and-white film or paper. Silver bromide is the workhorse of most photographic emulsions because it is more sensitive, while silver chloride is also genuinely used, especially in slower printing papers, which is why both answers are defensible and the key graced them.

• (a)AgF — Silver fluoride is highly soluble in water, unlike the other silver halides, and is not light-sensitive in the way photography needs, so it is not used to form the image-recording emulsion.
• (d)Ag₂SO₄ — Silver sulphate is not a silver halide and does not undergo the useful light-driven decomposition to metallic silver, so it plays no role in capturing a photographic image.

Black-and-white photography relies on the light sensitivity of silver halides. A film emulsion holds tiny crystals of silver bromide, often with some silver chloride and silver iodide, suspended in gelatin. Where light falls, the halide decomposes and specks of black silver form, so the exposed areas turn dark and an image is stored.

The sorting rule is: only a light-sensitive silver halide works. Two of the four options, silver bromide and silver chloride, are such halides, and UPSC accepted both. Silver fluoride is too soluble and silver sulphate is not a halide, so both are ruled out.

• Photographic emulsions use silver halides, chiefly silver bromide (AgBr), often with silver chloride (AgCl) and silver iodide (AgI).
• Light decomposes the halide and deposits grains of black metallic silver, forming the image.
• Silver chloride and silver bromide are only slightly soluble, unlike the very soluble silver fluoride.
• The 'fixer', sodium thiosulphate (hypo), later dissolves the unexposed halide so the picture no longer changes in light.

• Picking silver fluoride or silver sulphate by pattern-matching 'silver compound' without checking that it is a light-sensitive halide.
• Assuming only one option can be right when the official key here accepted two.

Asked as which compound records the image in black-and-white photography; look for a light-sensitive silver halide (bromide or chloride), not just any silver salt.

• practice — not a real PYQ

  The image on black-and-white photographic film is formed by grains of

  • (a)metallic silver
  • (b)silver oxide
  • (c)silver nitrate
  • (d)carbon

  Answer(a) metallic silver — light decomposes the silver halide, leaving dark specks of silver.

• practice — not a real PYQ

  Which one of the following is used as a 'fixer' in black-and-white photography?

  • (a)Sodium chloride
  • (b)Sodium thiosulphate
  • (c)Silver bromide
  • (d)Potassium nitrate

  Answer(b) Sodium thiosulphate — the hypo fixer dissolves the unexposed silver halide.