UPPSC 2021 Prelims GS Paper-I · Question 126 of 150

Correct — D, Crookes glass. Crookes glass is optical glass doped with cerium (and other rare-earth oxides) that strongly absorbs ultraviolet light. Sir William Crookes developed it in 1913 while looking for a lens that would protect glass-workers' eyes; the well-known Crookes tints absorb ultraviolet below about 350 nm, which is why the glass is used in protective and sun-glare eyewear. So it is the only glass here that 'cuts off' UV rays.

• (a)Soda glass — Ordinary soda-lime (soda) glass — the cheap glass of windows and bottles — transmits most near-ultraviolet and has no special UV-blocking additive, so it does not cut off UV rays.
• (b)Pyrex glass — Pyrex is a borosilicate glass prized for low thermal expansion (heat and thermal-shock resistance) for cookware and lab glassware; that property has nothing to do with absorbing ultraviolet.
• (c)Jena glass — Jena glass is another borosilicate/heat-resistant laboratory glass (from Schott, Jena); like Pyrex it is chosen for chemical and thermal durability, not for UV absorption.

Different 'special' glasses are defined by what they add to the basic silica–soda–lime melt. Adding boron oxide gives borosilicate glass (Pyrex, Jena) with very low thermal expansion. Adding cerium and other rare-earth oxides gives Crookes glass, whose electrons absorb ultraviolet photons, so the glass blocks UV while staying clear to visible light — the principle behind protective and sun glasses.

The trap is that Pyrex and Jena are the famous 'special' glasses students remember, so they get picked. But their fame is for heat resistance, not UV. The tell is the name Crookes — associated with the cerium-doped, UV-absorbing lens.

• Crookes glass = cerium (rare-earth) doped glass that absorbs ultraviolet radiation.
• Sir William Crookes developed it (1913) as protective eyewear; laid the basis of the modern sunglasses industry.
• Pyrex and Jena glass are borosilicate glasses valued for low thermal expansion / heat resistance, not UV blocking.
• Soda (soda-lime) glass is ordinary window/bottle glass and transmits near-UV.

• Picking Pyrex/Jena because they are the famous 'special' glasses — their property is heat resistance, not UV blocking
• Assuming ordinary glass already stops all UV (it blocks far-UV but passes near-UV)

UPSC/UPPSC science asks 'which glass cuts off UV rays' or matches a glass type to its defining property (Pyrex → heat resistance, Crookes → UV absorption).

• practice — not a real PYQ

  Crookes glass owes its ultraviolet-absorbing property mainly to the presence of which element?

  • (a)Cerium
  • (b)Lead
  • (c)Boron
  • (d)Sodium

  Answer(a) Cerium — a rare-earth oxide added to the glass that absorbs UV.

• practice — not a real PYQ

  Borosilicate glasses such as Pyrex and Jena glass are valued mainly for their:

  • (a)High ultraviolet absorption
  • (b)Low thermal expansion (heat resistance)
  • (c)High electrical conductivity
  • (d)Magnetic properties

  Answer(b) Low thermal expansion — they resist thermal shock, so are used for cookware and lab glassware.