CAPF AC 2022 Q22: Science & Technology – Physics
Which one of the following materials cannot be used to make a convex lens?
- (a) Aluminium
- (b) Glass
- (c) Sapphire
- (d) Water
Answer: (a) Aluminium
Explanation
What a lens must be made of
How to think: a lens works by refraction.
Light must pass through it and bend.
So the material must be transparent.
| Material | Transparent? | Can it make a lens? |
|---|---|---|
| Glass | Yes | Yes, the usual material |
| Sapphire | Yes | Yes; used for watch crystals and camera lens covers |
| Water | Yes | Yes; a water drop acts as a convex lens |
| Aluminium | No, it is an opaque metal | No |
What aluminium can do: it reflects light very well.
So it is used to coat mirrors, including large telescope mirrors.
Answer: (a) Aluminium.
Why each option is right or wrong
- (a) CORRECT. Aluminium is opaque, so light cannot pass through it to be refracted.
- (b) Incorrect. Glass is the common transparent material for lenses.
- (c) Incorrect. Sapphire is transparent and is used for tough lenses.
- (d) Incorrect. Water is transparent; a drop of water behaves like a convex lens.
Key point
A lens is a piece of transparent material bound by two surfaces, at least one of them spherical. It works by refraction. Opaque shiny materials such as aluminium and silver make mirrors, which work by reflection.
Remember
A lens needs a transparent material.
Common mistakes
- Thinking water cannot form a lens because it is a liquid.
- Not knowing that sapphire is a clear crystal of aluminium oxide.
- Mixing up a lens (refraction) with a mirror (reflection).
Exam tip
Educated guess: ask a simple question of each option: can I see through it? You can see through glass, sapphire and water, but not through a metal sheet.
Sources
- NCERT Class X Science — Light: Reflection and Refraction (refraction by spherical lenses)
- NCERT Class XII Physics — Ray Optics and Optical Instruments