In optical instruments such as spectacles, cameras and microscopes, the lens is commonly made from which special type of optical glass?

Introduction / Context:
Lenses are core components of many optical devices, including spectacles, cameras, microscopes and telescopes. They must be made from a type of glass that has well controlled optical properties such as refractive index and dispersion. This question checks whether you know the typical type of glass used to manufacture high quality lenses and introduces you to the idea that not all glass is the same from an optical point of view.

Given Data / Assumptions:

• The object in question is a lens used in optical instruments.
• Several kinds of glass are listed: ordinary window glass, Pyrex glass, flint glass and cobalt glass.
• We assume we want good transparency, suitable refractive index and controllable dispersion for focusing light.
• The task is to identify the glass type most commonly associated with lens making in basic general knowledge questions.

Concept / Approach:
Optical glasses are broadly classified into crown glass and flint glass families. Flint glass is a type of optical glass with relatively high refractive index and significant dispersion due to the presence of lead oxide or similar components. These characteristics make flint glass useful in lens systems where designers combine different types of glass to correct chromatic aberration. Ordinary window glass and coloured glasses are not optimised for such optical performance. Pyrex is formulated for thermal resistance and is commonly used in laboratory glassware and cooking utensils, not as the main material for precision lenses.

Step-by-Step Solution:
Step 1: Recognise that lens glass must be optically clear and have a controlled refractive index.Step 2: Recall that optical engineers often use flint glass as one of the standard lens materials.Step 3: Note that ordinary window glass is not designed with precise dispersion properties in mind.Step 4: Remember that Pyrex glass is famous for being heat resistant, not for special optical qualities required in lenses.Step 5: Cobalt glass is simply glass coloured by cobalt compounds and is mostly decorative.Step 6: Conclude that flint glass is the most appropriate answer among the given options for lens manufacturing.

Verification / Alternative check:
In basic optics references, you will often see the terms crown glass prism and flint glass prism. Achromatic doublets, which are lens combinations designed to reduce colour fringing, usually pair crown and flint glasses because of their differing refractive indices and dispersions. This repeated use of flint glass in standard lens design supports the choice made here.

Why Other Options Are Wrong:
Ordinary window glass is inexpensive and transparent but does not offer the carefully controlled properties needed for precision lenses. Pyrex glass is tailored for high thermal shock resistance and is widely used in beakers and ovenware rather than in optical lenses. Cobalt glass is mainly used for coloured decorative items or filters and is not the standard base for lens manufacturing. Therefore these options do not match the requirement as closely as flint glass does.

Common Pitfalls:
Students may be tempted by Pyrex because it sounds technical or by ordinary glass because it seems commonly available. Others may think coloured glasses like cobalt glass are used in fancy instruments. To avoid confusion, remember that flint glass is a textbook example of optical glass used in lenses and prisms due to its optical properties, not just its availability or appearance.

Final Answer:
Lenses in many optical instruments are commonly made from flint glass.