Which Compound is known as night glowing pigment from below?

Question

UnlockIAS · Accepted Answer

Correct — A, Europium dopped strontium aluminate — the booklet's spelling of europium-doped strontium aluminate. The material is strontium aluminate, SrAl₂O₄, activated with europium in its +2 state and co-doped with dysprosium, written SrAl₂O₄:Eu²⁺,Dy³⁺. It is the pigment behind virtually every glow-in-the-dark emergency exit sign, escape-route marking, watch dial and luminous toy made since the mid-1990s. The europium ion does the emitting; the dysprosium does the remembering. Daylight or room light in the 200–450 nm band lifts electrons out of Eu²⁺, the dysprosium-induced lattice defects trap that charge, and ordinary room-temperature heat then leaks it back to the europium over minutes and hours, each returning electron releasing a green photon peaking at about 520 nm. That stored-and-slowly-released step is persistent luminescence — phosphorescence, not fluorescence — and it is why the pigment keeps working in a blacked-out stairwell. The material has a datable origin: it was developed in 1993, the discovery credited to Yasumitsu Aoki of the Japanese firm Nemoto & Co., and patented by that company in 1994 as US Patent 5,424,006, 'Phosphorescent phosphor'. The motive was safety — the glow paints it replaced used radioactive promethium. Measured against the older copper-activated zinc sulphide it is roughly ten times brighter, glows about ten times longer and costs about ten times more, and it is now the brightest and longest-lasting phosphorescent material available commercially, sold under names such as Super-LumiNova and Seiko's Lumibrite. Crucially it stores nothing but light: strontium aluminate is non-toxic, chemically inert and endlessly rechargeable, unlike the radium dial paints and tritium tubes that preceded it.

Why the other options are wrong — (b) The cleverest distractor on the page, because it sits one swap away from something entirely real. The standard glow-in-the-dark pigment before 1993 was copper-activated zinc SULPHIDE, Cu:ZnS — the greenish material in old glow-in-the-dark stars and toys. This option moves the sulphide onto the copper and hands the host job to zinc oxide, a white pigment used in sunscreen and rubber. Copper sulphide is a dark semiconducting mineral, and the pairing has no persistent-luminescence use at all.  (c) Two genuinely useful compounds put to work in the wrong job. Barium sulphate is the inert white filler of paints and paper and the 'barium meal' swallowed for X-ray contrast; basic copper carbonate is malachite, a green mineral pigment. Neither phosphoresces, and the chemistry rules it out on principle: a persistent phosphor needs a colourless wide-band-gap host, whereas the Cu²⁺ ion is coloured precisely because it absorbs visible light, so it quenches emission rather than producing it.  (d) Boron oxide, B₂O₃, is a glass-former and flux — the borosilicate in laboratory glassware, the bead in the borax bead test — while copper sulphate is the blue crystalline salt of Bordeaux mixture and electroplating baths. The same fatal objection applies: copper(II) is intensely coloured and quenches luminescence, and neither compound serves as a phosphor host. Boron does appear in real strontium-aluminate phosphors, but only as a trace additive that lengthens the afterglow — never as a dopant for a copper salt.

Concept: A glowing pigment is never a single substance; it is a two-part design — a transparent host crystal plus a trace of a deliberately introduced impurity called the activator. Doping means seeding that impurity ion into the lattice at the level of a fraction of a per cent, the same idea as doping silicon with phosphorus, except that the payoff is optical rather than electrical. In SrAl₂O₄:Eu²⁺,Dy³⁺ the host is strontium aluminate, the activator is europium(II), and dysprosium(III) emits nothing at all: its role is to create lattice defects that act as electron traps. Incoming light excites the europium, the traps capture and hold part of the freed charge, and thermal energy releases it back gradually. Two things follow that examiners love. First, the oxidation state is not decoration — Eu²⁺ gives a strong afterglow while Eu³⁺ gives almost none. Second, the delay is what distinguishes the vocabulary. Fluorescence ceases within nanoseconds of the exciting light being switched off, as with a highlighter pen under a UV lamp; phosphorescence, more precisely persistent luminescence, runs on for seconds to hours because the energy sits in traps. Keep a third mechanism separate from both: retroreflective road studs and cats' eyes look bright at night but emit nothing of their own — they simply bounce headlight beams straight back at the driver.

State PCS 2025 Q132 — Science & Technology PYQ with Answer

State PCS 2025 Q132 — Science & Technology PYQ with Answer