Which of the following materials cannot be used for making optical lenses because it is not transparent?

Introduction / Context:

This question belongs to basic optics and materials. Lenses are transparent optical elements that refract light to form images. To function as a lens, a material must allow light to pass through it with minimal scattering and absorption. Understanding which materials can or cannot be used for lenses reinforces the idea that transparency is essential for image formation.

Given Data / Assumptions:

• The question asks about materials used for making lenses.
• Options given are plastic, soil, glass, and water.
• We assume normal visible light and everyday optical lenses.

Concept / Approach:

Lenses require materials that are transparent so that light can pass through and be refracted at the surfaces to focus or diverge beams. Common lens materials include glass and various plastics, and even water can act as a lens in simple demonstrations because it is transparent. Soil, however, is an opaque and heterogeneous mixture of particles and cannot transmit light clearly, so it cannot be shaped into a functional optical lens.

Step-by-Step Solution:

Verification / Alternative check:

In practical life, we see glass lenses in cameras and spectacles, plastic lenses in inexpensive optics and sunglasses, and water drops acting as small lenses on surfaces. No optical devices are made from soil, because it does not permit the formation of clear images. Simple experiments with a spoonful of soil show that it blocks light instead of transmitting it, contrasting sharply with the behaviour of glass or clear plastic.

Why Other Options Are Wrong:

• Plastic: Transparent plastics like acrylic and polycarbonate are commonly moulded into lenses.
• Glass: Traditional and high quality optical lenses are made from specially formulated glass.
• Water: Although not used in permanent instruments, water droplets and containers of water can act as lenses due to transparency and refraction.

Common Pitfalls:

Students sometimes focus on the solid or liquid state of a substance rather than its optical properties. They may assume only solids can be lenses, overlooking water lenses. The critical property is transparency and refractive index, not whether the material is a solid or liquid. Soil is a mixture of opaque particles and does not meet this requirement. Remember: if light cannot pass through a material, it cannot be used as a lens in the usual optical sense.

Final Answer:

Soil cannot be used for making lenses because it is not transparent.