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Indian Forest Service (Mains)

Chemistry optional

  • 2 papers
  • 200 marks each
  • 3 h per paper

Paper I covers physical and inorganic chemistry.

Indian Forest Service (Mains), Chemistry optional · 2025 · Paper I

Q6(a)(ii). Fluorescence and phosphorescence

Question

Why do fluorescence and phosphorescence normally have lower frequencies than the absorbed radiation?

Answer in brief

Excited molecules lose energy before radiative emission, so the emitted photons normally have lower energy and frequency.

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The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

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Topic notes: photochemistry (quantum yield, Jablonski diagram, H₂–halogen reactions), with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversfluorescencephosphorescenceStokes shiftJablonski diagramintersystem crossing

All parts of Question 6