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

A lens is a transmissive optical device that focuses or disperses light by means of refraction. To function as a lens, a material must be transparent (or at least translucent) to allow light rays to pass through it.

• Water, Glass, and Plastic: These are transparent materials. Light can pass through them and bend according to their refractive index, enabling them to act as lenses. For example, water droplets can magnify text, and most modern eyeglasses use specialized plastics.
• Clay: Clay is an opaque material. It does not allow light to pass through it; instead, light is either reflected or absorbed. Since light cannot be transmitted or refracted through clay, it cannot be used to make a lens.