physics

Petrographic microscope

Petrographic microscope is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Petrographic microscope rather than just read about it. In short: A petrographic microscope is a type of optical microscope used to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks.

Key takeaways

Petrographic microscope belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Petrographic microscope to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Petrographic microscope from memory before moving on to harder problems.

Reference excerpt

A petrographic microscope is a type of optical microscope used to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks. The method includes aspects of polarized light microscopy (PLM).

Description Depending on the grade of observation required, petrographic microscopes are derived from conventional brightfield microscopes of similar basic capabilities by:

Adding a Nicol prism polarizer filter to the light path beneath the sample slide Replacing the normal stage with a circular rotating stage (typically graduated with vernier scales for reading orientations to better than 1 degree of arc) Adding a second rotatable and removable Nicol prism filter, called the analyzer, to the light path between objective and eyepiece Adding a phase telescope, also known as a Bertrand lens, which allows the viewer to see conoscopic interference patterns Adding a slot for insertion of wave plates Petrographic microscopes are constructed with optical parts that do not add unwanted polarizing effects due to strained glass, or polarization by reflection in prisms and mirrors. These special parts add to the cost and complexity of the microscope. However, a "simple polarizing" microscope is easily made by adding inexpensive polarizing filters to a standard biological microscope, often with one in a filter holder beneath the condenser, and a second inserted beneath the head or eyepiece. These can be sufficient for many non-quantitative purposes. The two Nicol prisms (occasionally referred to as nicols) of the petrographic microscope have their polarizing planes oriented perpendicular to one another. When only an isotropic material such as air, water, or glass exists between the filters, all light is blocked, but most crystalline materials and minerals change the polarizing light directions, allowing some of the altered light to pass through the analyzer to the viewer. Using one polarizer makes it possible to view the slide in plane polarized light; using two allows for analysis under cross polarized light. A particular light pattern on the upper lens surface of the objectives is created as a conoscopic interference pattern (or interference figure) characteristic of uniaxial and biaxial minerals, and produced with convergent polarized light. To observe the interference figure, true petrographic microscopes usually include an accessory called a Bertrand lens, which focuses and enlarges the figure. It is also possible to remove an eyepiece lens to make a direct observation of the objective lens surface. In addition to modifications of the microscope's optical system, petrographic microscopes allow for the insertion of specially-cut oriented filters of biaxial minerals (the quartz wedge, quarter-wave mica plate and half-wave mica plate), into the optical train between the polarizers to identify positive and negative birefringence, and in extreme cases, the mineral order when needed.

History As early as 1808, the French physicist Étienne Louis Malus discovered the refraction and polarization of light. William Nicol invented a prism for polarization in 1829, which was an indispensable part of the polarizing microscope for over 100 years. Later the Nicol prisms were replaced by cheaper polarizing filters. The first complete polarizing microscope was built by Giovanni Battista Amici in 1830. Rudolf Fuess built the first polarization microscope specifically for petrographic purposes in 1875. This was described by Harry Rosenbusch in the yearbook for mineralogy.

References

Illustrations

Petrographic microscope: Leica DMRX incident light microscope with mechanical stage and Swift F automated point counter for analysis of organic composition of coal and rock samples — Leica DMRX incident light microscope with mechanical stage and Swift F automated point counter for analysis of organic composition of coal and rock samples

Worked examples

Example 1 — a first encounter with Petrographic microscope

Start with the simplest possible case. Write down what Petrographic microscope claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Petrographic microscope before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Petrographic microscope ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Petrographic microscope

In research: Petrographic microscope appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Petrographic microscope in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Petrographic microscope is common in secondary-school and first-year university syllabi. It links to neighbouring topics Microscopes, Optical mineralogy, so understanding it makes those chapters shorter.
In everyday life: Look for Petrographic microscope outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Petrographic microscope in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Petrographic microscope means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Petrographic microscope out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Petrographic microscope in simple terms?

Why does Petrographic microscope matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Petrographic microscope?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Petrographic microscope.