UPSC Prelims 2007 Q112: Science & Technology – Materials & Engineering
Which one of the following types of glass can cut off ultraviolet rays?
- (a) Soda glass
- (b) Pyrex glass
- (c) Jena glass
- (d) Crookes glass
Answer: (d) Crookes glass
Explanation
Types of glass and their uses
| Glass | Key ingredient / property | Use |
|---|---|---|
| Crookes glass | Contains cerium oxide — absorbs UV rays | Sunglasses / spectacle lenses |
| Pyrex (borosilicate) | Low thermal expansion — handles heat | Lab glassware, ovenware |
| Jena glass | Borosilicate type — chemical and heat resistant | Lab apparatus |
| Soda (soft) glass | Sodium + calcium silicate | Windows, bottles |
| Flint glass | Lead oxide — high refractive index | Lenses, prisms |
Answer: (d) Crookes glass.
Why each option is right or wrong
- (a) Incorrect. Soda glass is ordinary window and bottle glass.
- (b) Incorrect. Pyrex resists heat, not UV.
- (c) Incorrect. Jena glass is for laboratory use — heat and chemical resistant.
- (d) CORRECT. Crookes glass (with cerium oxide) cuts off UV — used in sun goggles.
Key point
Crookes glass (cerium oxide) = UV protection. Pyrex/Jena = heat resistant. Flint = lenses. Soda = windows.
Remember
Crookes glass cuts off ultraviolet rays.
Common mistakes
- Choosing Pyrex because it is the best-known 'special' glass.
- Confusing Crookes glass with the Crookes tube (cathode-ray tube).
- Thinking all tinted glass blocks UV — the cerium oxide is what does it.
Exam tip
Crookes → 'Cuts UV' → sunglasses. Pyrex → kitchen and lab heat.
Sources
- Encyclopaedia Britannica — glass
- Encyclopaedia Britannica — ultraviolet radiation