Petrographic Microscope
As our final project, we decided to build a petrographic microscope. A petrographic microscope is a specialized compound microscope used by geologists to examine thin sections of rocks and other translucent materials. It uses polarized light to reveal optical properties of minerals, which enables identification and provides information on rock forming processes.
Key Components
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Polarizing filters: A polarizer is placed beneath the specimen to polarize the light, while an analyzer is placed above/after the objective lens to interact with the polarized light. The two filters are perpendicular to each other.
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Rotating Stage: The rotating stage allows the user to precisely position and orient the specimen.
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Substage condenser: A high quality condenser concentrates the light and can be adjusted for parallel/convergent illumination
How it works
1. Light is polarized by the polarizer before it passes through the thin rock section.
2. As the polarized light passes through the mineral crystals in the rock sample, it’s properties change based on the crystal’s optical characteristics.
3. When the light reaches the analyzer, it’s interaction with the crystal is revealed, allowing for the identification of the mineral and it’s relationship to surrounding minerals.
Isotropic
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have the same optical properties in all directions due to their symmetric cubic crystal structure
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light passes through at a single refractive index
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transparent but appear black when polarizers are crossed
vs
Anisotropic
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have direction-dependent properties
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splits plane-polarized light into two rays (fast and slow) with different refractive indices which produces birefringence (double refraction)
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go through a cycle of colors as filters rotate and then turn black (go extinct) at 90 degrees perpendicular
Minerals
Our Model / Components
5cm
5cm
7cm
*
5.5cm
12cm
40cm
LED Light Source
Condenser (convex)
f= 50
Polarizer
Aperature
Thin section
Objective (convex)
f= 50
Polarizer
Image
Calculations
The optical components that were located before our thin section of rock didn't require any calculations. To determine the distance required between the object and lens, we used an arbitrary image distance. We set our screen 40cm from the second polarizing filter, creating an image distance of 52cm from lens 2. We used the thin lens equation to solve for the object distance and determined that our thin section of rock needed to be 5.5cm in front of lens 2 in order to produce the proper image.
Result
We were successful with generating an image! When the image looks brightest, both polarizing lenses are aligned together and letting the most light through. As we rotated the analyzer 90 degrees, dark spots appeared. This is called extinction, when the mineral's internal vibration directions align with those of the upper and lower polarizers. The bright spots are caused by plane-polarized light hitting anisotropic minerals. The LED light was split into two separate waves that travel at different velocities. The recombination of these rays is what causes interference colors. However, our's appeared as white light. This is likely due to our sample not having any anisotropic minerals or because our slide was not completely at a 45 degree angle where birefringence is most optimized.
Optics Experimental Review
What did we learn?
Building a petrographic microscope teaches you a lot about optics because it relies on core principles of how light behaves. You learn how polarization works, why minerals change the vibration direction of light, and how interference can create the colorful patterns seen under crossed polars. You also gain hands-on understanding of refraction and lenses, which shows how microscopes produce clear, evenly lit images. Overall, building the microscope becomes a direct, real-world lesson in both geometric and wave optics.