Which one among the following is used in the manufacture of ultra-violet protective glasses?

Correct — D, Cerium oxide. Cerium is a rare-earth element, and cerium oxide added to a glass melt absorbs strongly in the ultra-violet while leaving the visible band almost untouched. That combination is exactly what a protective glass needs: it must stop the harmful short wavelengths without darkening. Cerium-doped glass is used in welding and laboratory eyewear, in glass for display cases and windows where fading has to be prevented, and in the cover glasses of spacecraft solar cells, where it also stops the glass browning under radiation. The same oxide is used to decolourise glass, because it oxidises the iron impurity that otherwise turns the batch green.

• (a)Aluminium oxide — Aluminium oxide is a stabiliser in glass. It raises chemical durability and resistance to attack by water, but it does not absorb ultra-violet radiation selectively.
• (b)Tungsten oxide — Tungsten oxide is used in electrochromic or smart glass, which darkens under an applied voltage. That effect is switchable and covers the visible band; it is not the fixed ultra-violet filter the question describes.
• (c)Molybdenum oxide — Molybdenum oxide is used as a catalyst and in pigments, and has no role as an ultra-violet absorber in protective glass.

Ultra-violet radiation carries enough energy per photon to break chemical bonds, which is what causes sunburn, fading of dyes and embrittlement of plastics. A protective glass has to absorb wavelengths shorter than about 400 nanometres while transmitting the visible band from about 400 to 700 nanometres. Additives whose electronic transitions fall in the ultra-violet do exactly this, and cerium oxide is the standard one; titanium and iron oxides also absorb ultra-violet but tint the glass more.

Three of the four options are oxides with real industrial uses, so the item cannot be solved by rejecting unfamiliar names. The way in is the rare-earth connection: cerium is the most abundant of the rare earths and is the element associated with ultra-violet absorption and with glass polishing, where cerium oxide powder is the standard abrasive. Ordinary window glass already blocks most ultra-violet B; it is ultra-violet A that needs a deliberate absorber, and that is what cerium doping supplies.

• Cerium is the most abundant rare-earth element, and cerium oxide is its common industrial form.
• Cerium oxide absorbs ultra-violet strongly while transmitting visible light, which is what a protective glass needs.
• The same oxide is used as a decolouriser in glass and as the standard glass-polishing powder.
• Cerium-doped cover glass protects spacecraft solar cells from radiation browning.
• Ordinary window glass already stops most ultra-violet B; the deliberate additive is aimed at ultra-violet A.

• Choosing tungsten oxide because smart glass is the more familiar technology.
• Assuming all glass additives that block light must also darken it.
• Confusing the polishing use of cerium oxide with its optical use, though both are real.

A one-name materials item. Three plausible industrial oxides are offered, and the rare-earth connection is the discriminator.

• practice — not a real PYQ

  Cerium oxide is widely used in the glass industry as a

  • (a)polishing powder
  • (b)fluxing agent
  • (c)network former
  • (d)refractory lining

  Answer(a) polishing powder — it is the standard abrasive for finishing optical glass.

• practice — not a real PYQ

  Which layer of the atmosphere shields the Earth from most ultra-violet radiation?

  • (a)Troposphere
  • (b)Stratosphere
  • (c)Mesosphere
  • (d)Thermosphere

  Answer(b) Stratosphere — the ozone layer lies within it.