Optical lenses used in scientific instruments are usually manufactured from which special type of glass?

Introduction / Context:
Lenses in optical instruments such as microscopes, cameras and telescopes must be made from materials with suitable optical properties. Different types of glass vary in how they bend and disperse light. This question tests whether you know that flint glass, not ordinary or coloured glass, is commonly used for high quality lenses.

Given Data / Assumptions:

• Optical lenses need a high refractive index and controlled dispersion.
• Glass types include Pyrex, flint glass and ordinary soda lime glass.
• Cobalt glass is mainly used for colour, not for precision optics.

Concept / Approach:
Flint glass is a type of glass with a relatively high refractive index and significant dispersion due to the presence of lead oxide or similar components. These properties allow lens makers to design combinations of lenses that reduce optical aberrations and improve image quality. Ordinary soda lime glass is cheaper but has less favourable optical properties for precision lenses. Pyrex is valued for heat resistance in laboratory glassware, and cobalt glass is mainly used as a coloured glass for filters and decorative items.

Step-by-Step Solution:
Step 1: Recall that lenses must bend light efficiently and can be combined to correct aberrations. Step 2: Recognise that flint glass has a high refractive index, which helps in lens design. Step 3: Note that Pyrex glass is primarily heat resistant and used in beakers and test tubes, not in precision lenses. Step 4: Understand that ordinary soda lime glass is common in windows and bottles but not ideal for high quality optical lenses. Step 5: Confirm that cobalt glass is coloured and used mainly as a filter or decorative material, not as the main lens material, and therefore choose flint glass.

Verification / Alternative check:
Optics textbooks and engineering references describe crown glass and flint glass as two main categories of optical glass. Flint glass types are used in components of compound lenses to manage chromatic aberration. Laboratory equipment catalogues list Pyrex glassware separately from optical components, confirming their different uses. This supports the conclusion that flint glass is the correct choice for lenses.

Why Other Options Are Wrong:
Option a, Pyrex glass, is specially formulated to withstand heat and sudden temperature changes and is widely used in laboratory flasks and kitchenware, not as standard lens material.

Option c, ordinary soda lime glass, is common in windows and bottles but lacks the precise optical properties desired for scientific instruments.

Option d, cobalt coloured glass, is mainly used for its blue colour and can serve as a filter but is not the usual base material for lenses themselves.

Common Pitfalls:
Students sometimes select Pyrex because they recognise it as scientific glass, forgetting that lenses require specific optical qualities rather than just heat resistance. Another pitfall is to choose ordinary glass because it seems most common, but common use does not always indicate suitability for precision optics. Focusing on refractive index and dispersion leads to the correct answer.

Final Answer:
Optical lenses are usually made from Flint glass because of its favourable refractive and dispersion properties.