Which one of the following materials cannot be used to make a convex lens?

A lens is defined as a piece of transparent material with curved surfaces that can converge or diverge light. For a material to function as a lens, it must allow light to pass through it (transparency). Glass and plastic are the most common materials used for this purpose. Water can also act as a simple lens, as seen in curved water drops that magnify objects. Sapphire, a single-crystal form of aluminium oxide (Al2O3), is highly transparent and is frequently used to manufacture precision plano-convex lenses for high-performance optical systems [1]. In contrast, aluminium in its metallic form is opaque and does not transmit visible light, making it impossible to use as a material for a refractive convex lens. While sapphire is chemically related to aluminium, the metallic element itself lacks the necessary optical transparency required for lens fabrication.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 10: Light: Mirrors and Lenses > Activity 10.8: Let us explore > p. 162