Question

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

• A. Water
• B. Glass
• C. Clay
• D. Plastic

Hint:

The primary requirement for a material to make a lens is that it must be transparent to the light it is intended to manipulate. Opaque materials, like clay, block light and cannot form lenses. Common lens materials like glass and plastic are chosen for their transparency and refractive properties.

Answer 1

The Correct Option is C

Concept: A lens is an optical device that transmits and refracts light, converging or diverging the beam. To function as a lens, a material must primarily be transparent or translucent to the type of radiation it is intended for (usually visible light). Step 1: Properties required for a lens material For a material to be used to make a lens for visible light, it should ideally have the following properties:
Transparency: It must allow light to pass through it with minimal scattering or absorption.
Refractive Index: It must have a refractive index different from the surrounding medium (usually air) to be able to bend light.
Homogeneity: It should be optically uniform.
Workability: It should be possible to shape it into the curved surfaces required for a lens.
Durability and Stability: It should be reasonably hard, stable, and not degrade easily. The most critical property for basic function is transparency. Step 2: Evaluate the given materials
(1) Water: Water is transparent to visible light. It has a refractive index (\(\approx 1.33\)) different from air. Liquid lenses exist, and water can be contained in a shaped transparent container to act as a lens (e.g., a spherical flask filled with water can act as a magnifying lens). So, water can be used.
(2) Glass: Glass is a very common material for making lenses due to its excellent transparency, workability, and stability. Various types of optical glass are specifically designed for lens manufacturing. So, glass can be used.
(3) Clay: Clay is an opaque material. It does not allow visible light to pass through it. Therefore, it cannot refract light in the way required for a lens to form an image. So, clay cannot be used.
(4) Plastic: Many types of plastics (e.g., acrylic, polycarbonate) are transparent and are widely used to make lenses, especially for eyeglasses, contact lenses, and inexpensive optical instruments. They are lightweight and shatter-resistant. So, plastic can be used. Conclusion: Among the given options, clay is the only material that is opaque and thus cannot be used to make a functional lens for visible light. Therefore, clay cannot be used to make a lens.