UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II
Q1(p). Why watch hands glow in the dark
Question
How do watch hands glow in the dark?
Answer in brief
By phosphorescence. The paint stores absorbed light in a long-lived excited state and emits it slowly.
Key points
- The hands carry a phosphor: ZnS:Cu, or the modern SrAl₂O₄:Eu,Dy.
- Light excites it. Intersystem crossing takes the energy to a triplet state; in the solid, electrons are held in trap levels.
- Return to the ground state is spin-forbidden ⇒ slow emission for minutes to hours: phosphorescence.
- Fluorescence stops at once, so it cannot give an afterglow.
- Some dials use radioluminescence: ZnS excited by tritium (formerly ²²⁶Ra) glows without light charging.
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.
CoversphosphorescenceJablonski diagramintersystem crossingfluorescenceradioluminescence
All parts of Question 1
- 1(a) Why Z > 1 for H₂ and He at all pressures5 marks
- 1(b) Corresponding state of H₂S for N₂ at 25 °C and 1 atm5 marks
- 1(c) Band structure of n-type and p-type semiconductors5 marks
- 1(d) Symmetry elements of orthorhombic Mg₂SiO₄5 marks
- 1(e) Fall of G with temperature at constant pressure5 marks
- 1(f) Z > 1 and f > P for the gas P(Vm − b) = RT5 marks
- 1(g) Overall equation and the role of each species5 marks
- 1(h) log k against pH for specific H⁺ catalysis5 marks
- 1(i) KCl salt bridge for a silver–lead cell5 marks
- 1(j) Equilibrium constant from standard potentials5 marks
- 1(k) Conditions for an acceptable wave function5 marks
- 1(l) No solution with E = 0 for the particle in a box5 marks
- 1(m) Discrete levels and the resonance condition5 marks
- 1(n) Rotational activity of CO and CO₂5 marks
- 1(o) Measuring light intensity with a chemical actinometer5 marks
- 1(p) Why watch hands glow in the dark5 marks