Gem_5106 Optical properties of gems polarization of light

• Nature of polarized light
• Optic axes
• Isotropic and anisotropic behavior
• The production of polarized light
• The polarizing filter
• Construction, uses and typically results of Polariscope
• Interference figures

Nature of polarized light in gemology

In gemology, polarized light is a crucial tool used to study the optical properties of gemstones. Here’s a detailed look at its nature and applications in this field:

Polarization of Light

Polarization refers to the orientation of the oscillations of the light waves. Unpolarized light, such as sunlight or light from a standard bulb, consists of waves vibrating in multiple planes. Polarized light, however, has waves vibrating in a single plane.

Creation of Polarized Light

Polarized light can be created using polarizing filters, which allow only light vibrating in a specific direction to pass through. Gemologists use polariscopes, which consist of two polarizing filters: a polarizer and an analyzer.

Applications in Gemology

1. Identifying Gemstone Species:

Isotropic vs. Anisotropic:

Isotropic Gems: These gems (e.g., garnet, spinel) have the same optical properties in all directions and do not affect polarized light.

Anisotropic Gems: These gems (e.g., quartz, tourmaline) have different optical properties depending on the direction of light travel and can split polarized light into two rays (double refraction).

2. Determining Crystal Structure:

Single Refraction: Observed in isotropic materials; the light remains unaffected.

Double Refraction: Seen in anisotropic materials; light splits into two rays with different velocities, which can be observed through interference patterns.

3. Identifying Inclusions and Strain Patterns:

Polarized light helps reveal internal features such as inclusions, strain patterns, and growth structures, which are critical for gemstone identification and determining origin.

4. Detecting Treatments and Enhancements:

Certain treatments and enhancements can alter a gemstone’s optical properties. Polarized light can help detect such modifications.

Equipment

1. Polariscope:

Consists of two polarized lenses with the gemstone placed in between. By rotating the gemstone, gemologists can observe changes in light patterns, indicating whether the gem is isotropic or anisotropic.

2. Refractometer:

Uses polarized light to measure the refractive index, which is a key property for identifying gemstones.

3. Conoscope:

Used in conjunction with a polariscope to examine interference patterns, aiding in the identification of optic sign and crystal system.

Practical Example

Consider a gemstone like tourmaline, which is anisotropic. When viewed under a polariscope, it will display different colors and brightness levels as it is rotated. This behavior contrasts with isotropic gems, which would not change appearance under polarized light.

Conclusion

Polarized light is an essential tool in gemology, offering insights into the optical properties and internal features of gemstones. By understanding how different gemstones interact with polarized light, gemologists can accurately identify, classify, and evaluate gemstones.

Optic axes

In gemology and crystallography, the concept of optic axes is fundamental for understanding the optical properties of anisotropic materials. Here’s a detailed explanation:

Optic Axes

Optic axes are directions in a crystal along which light can travel without experiencing double refraction (birefringence). In these directions, the refractive index is uniform, and light behaves similarly to how it would in an isotropic material.

Types of Crystals Based on Optic Axes

1. Uniaxial Crystals:

Characteristics: These crystals have a single optic axis.

Examples: Quartz, calcite, and tourmaline.

Optical Behavior: Light traveling along the optic axis does not split into two rays, but light entering at other angles will split into an ordinary ray (with constant velocity) and an extraordinary ray (with variable velocity depending on the direction).

Refractive Indices: Uniaxial crystals have two principal refractive indices: the ordinary refractive index (\( n_o \)) and the extraordinary refractive index (\( n_e \)).

2. Biaxial Crystals:

Characteristics: These crystals have two optic axes.

Examples: Topaz, orthoclase, and gypsum.

Optical Behavior: Light entering along either of the two optic axes will not experience double refraction. Light entering at other angles will split into two rays, both of which are extraordinary rays but with different velocities.

Refractive Indices: Biaxial crystals have three principal refractive indices: \( n_x \), \( n_y \), and \( n_z \).

Identifying Optic Axes

1. Polariscope and Conoscope:

Polariscope: Used to examine the optical behavior of a gemstone by placing it between two polarized lenses and observing changes in light patterns.

Conoscope: An accessory used with the polariscope to view interference figures, which help in identifying the optic axes and determining whether the crystal is uniaxial or biaxial.

2. Interference Figures:

Uniaxial Figures: Appear as a cross or a bull’seye pattern when viewed along the optic axis.

Biaxial Figures: Display two curved isogyres (black bars) that move and change shape as the stage is rotated, indicating the presence of two optic axes.

Practical Application

Determining Gemstone Identity: Knowing whether a gemstone is uniaxial or biaxial helps in its identification. For instance, if a gemstone shows a uniaxial interference figure, it narrows down the possibilities to uniaxial minerals.

Evaluating Optical Properties: Understanding the optic axes helps gemologists evaluate birefringence and other optical properties crucial for the gemstone’s quality and value assessment.

Example

Consider a piece of calcite, which is a uniaxial crystal. When viewed along its optic axis through a polariscope and conoscopic lens, it will exhibit a bull’seye interference pattern. If the same examination is done on a biaxial crystal like topaz, the interference figure will show two curved isogyres that move upon stage rotation.

 

Conclusion

 

Optic axes are key to understanding and identifying the optical characteristics of anisotropic materials. Recognizing whether a gemstone has one or two optic axes (uniaxial or biaxial) is vital for gem identification, evaluation of optical properties, and determination of crystal structure.

 

Isotropic and anisotropic behavior

 

In gemology and crystallography, understanding the behavior of light in isotropic and anisotropic materials is essential for identifying and characterizing gemstones. Here’s a detailed explanation of these behaviors:

 

Isotropic Behavior

 

Isotropic Materials:

Definition: Isotropic materials have the same optical properties in all directions. Light travels through these materials at a constant speed, regardless of the direction.

Examples: Garnet, spinel, glass, and cubic crystals (e.g., diamond).

Optical Characteristics:

Single Refractive Index: Isotropic materials have a single refractive index because light travels at the same speed in all directions.

No Birefringence: Since light is not split into different paths, there is no double refraction (birefringence).

Behavior Under Polarized Light: When observed under polarized light using a polariscope, isotropic materials remain dark as the stage is rotated, indicating no change in light behavior.

 

Anisotropic Behavior

 

Anisotropic Materials:

Definition: Anisotropic materials have different optical properties in different directions. Light traveling through these materials experiences different velocities depending on the direction.

Examples: Quartz, calcite, tourmaline, topaz, and most other noncubic crystals.

Optical Characteristics:

Multiple Refractive Indices: Anisotropic materials have more than one refractive index because the speed of light varies with direction.

Birefringence: These materials exhibit double refraction, where incoming light splits into two rays with different velocities and refractive indices.

Behavior Under Polarized Light: When observed under polarized light using a polariscope, anisotropic materials show changes in color and brightness as the stage is rotated. This indicates variations in light behavior due to different refractive indices.

 

Types of Anisotropic Crystals

 

1. Uniaxial Crystals:

Definition: Have a single optic axis.

Behavior: Light traveling along the optic axis behaves like it would in an isotropic material (no double refraction). In other directions, light splits into an ordinary ray and an extraordinary ray.

Examples: Quartz, calcite, tourmaline.

 

2. Biaxial Crystals:

Definition: Have two optic axes.

Behavior: Light traveling along either optic axis does not experience double refraction. In other directions, light splits into two rays, both extraordinary.

Examples: Topaz, orthoclase, gypsum.

 

Practical Implications in Gemology

 

Identification:

Isotropic and anisotropic behaviors are used to identify and differentiate between gemstone species.

A polariscope can help determine if a gemstone is isotropic or anisotropic by observing how it interacts with polarized light.

 

Evaluating Optical Properties:

Understanding the refractive indices and birefringence helps in assessing the optical quality of gemstones.

Anisotropic materials often show unique optical effects like pleochroism (different colors in different directions), which can be used for identification and evaluation.

 

Example of Anisotropic Behavior

 

Consider a piece of calcite, a uniaxial crystal. When light enters the calcite at an angle not aligned with the optic axis, it splits into two rays: an ordinary ray and an extraordinary ray. This double refraction can be observed as a double image when looking through the calcite.

 

Conclusion

 

Isotropic and anisotropic behaviors are fundamental concepts in gemology, crucial for identifying and characterizing gemstones. Isotropic materials have uniform optical properties in all directions, while anisotropic materials exhibit varying optical properties based on direction, leading to phenomena like double refraction and birefringence. Understanding these behaviors allows gemologists to accurately identify and evaluate gemstones.

 

The production of polarized light

 

Polarized light is produced by restricting the vibrations of light waves to a single plane. Here’s a detailed look at the various methods used to produce polarized light and their applications, particularly in gemology:

 

Methods of Producing Polarized Light

 

1. Polarization by Absorption (Using Polaroid Filters):

Mechanism: Polaroid filters consist of longchain polymer molecules aligned in a specific direction. These molecules absorb light vibrating in the direction parallel to their alignment and transmit light vibrating perpendicular to it.

Applications: Polaroid filters are commonly used in sunglasses, camera lenses, and scientific instruments like polariscopes.

 

2. Polarization by Reflection:

Mechanism: When light reflects off a nonmetallic surface at a specific angle (Brewster’s angle), the reflected light becomes polarized with its electric field perpendicular to the plane of incidence.

Applications: This principle is used in photography to reduce glare and in optical instruments to produce polarized light.

 

3. Polarization by Refraction (Double Refraction or Birefringence):

Mechanism: Certain crystals, like calcite and quartz, have different refractive indices along different axes. When light enters such a crystal, it splits into two rays polarized at right angles to each other, traveling at different speeds.

Applications: Double refraction is used in optical devices such as Nicol prisms to produce polarized light.

 

4. Polarization by Scattering:

Mechanism: When light passes through a medium containing small particles, it scatters. The scattered light becomes partially polarized, with the degree of polarization depending on the angle of scattering.

Applications: This phenomenon explains the blue color of the sky and is used in atmospheric studies.

 

Practical Devices for Producing Polarized Light

 

1. Polaroid Filters:

Description: Thin plastic sheets embedded with oriented molecules that absorb light polarized in one direction.

Use: Widely used in polarizing sunglasses, camera filters, and scientific instruments like polariscopes.

 

2. Nicol Prism:

Description: A type of polarizing prism made from calcite crystals, designed to split incoming light into two polarized beams and transmit only one.

Use: Historically used in optical instruments, though modern devices often use synthetic polarizing filters instead.

 

3. Brewster’s Angle Apparatus:

Description: An optical setup where light is reflected off a surface at Brewster’s angle to produce polarized light.

Use: Used in various optical experiments and instruments to produce and study polarized light.

 

4. Wave Plates (Retarders):

Description: Optical devices made from birefringent materials that introduce a phase difference between orthogonal components of light, converting linearly polarized light into circularly polarized light or vice versa.

Use: Used in optical instruments to control the state of polarization of light.

 

Application in Gemology

 

In gemology, polarized light is essential for examining and identifying gemstones. Here’s how polarized light is used:

 

1. Polariscope:

Function: A device consisting of two polarizing filters (a polarizer and an analyzer) with a rotating stage for the gemstone.

Use: Helps determine if a gemstone is isotropic or anisotropic. Anisotropic gems will show changes in light patterns as the gemstone is rotated.

 

2. Identifying Optical Properties:

Birefringence: Observing how light splits and behaves in anisotropic materials reveals information about birefringence, helping to identify the gemstone.

Pleochroism: Polarized light helps observe color changes in pleochroic gemstones, indicating different absorption of light in different crystallographic directions.

 

3. Detecting Inclusions and Strain:

Internal Features: Polarized light reveals internal inclusions, growth patterns, and strain within gemstones, aiding in identification and quality assessment.

 

Conclusion

 

The production of polarized light is a fundamental concept with various methods, including absorption, reflection, refraction, and scattering. In gemology, polarized light is crucial for identifying and analyzing gemstones, revealing their optical properties, internal features, and structural characteristics. Devices like polariscopes, Polaroid filters, and wave plates are essential tools in this process.

 

The polarizing filter

 

A polarizing filter is a device that produces polarized light by allowing light waves of a specific orientation to pass through while blocking others. This process is crucial in various applications, including photography, LCD screens, and gemology. Here’s a detailed look at the polarizing filter and its functions:

 

Structure and Function

 

How Polarizing Filters Work:

1. Polarization by Absorption:

Polarizing filters typically consist of a material that absorbs light waves vibrating in certain directions. Common materials include Polaroid, a type of synthetic plastic film.

The filter is made of longchain polymers aligned in one direction. These polymers absorb light waves vibrating parallel to their alignment while allowing waves vibrating perpendicular to pass through.

 

2. Types of Polarizing Filters:

Linear Polarizers: These filters allow light waves vibrating in a single direction to pass through. They are used in various optical instruments, including cameras and polariscopes.

Circular Polarizers: These filters consist of a linear polarizer combined with a quarterwave plate, which converts the linearly polarized light into circularly polarized light. They are often used in photography to reduce reflections and glare without affecting autofocus and metering systems in cameras.

 

Applications in Gemology

 

Polariscope:

A polariscope is a common tool in gemology that uses two polarizing filters: a polarizer and an analyzer.

Polarizer: The first filter, placed below the gemstone, polarizes the light before it enters the gemstone.

Analyzer: The second filter, placed above the gemstone, is rotated to analyze the light emerging from the gemstone.

 

Using Polarizing Filters in Gemology:

1. Identifying Isotropic and Anisotropic Materials:

Isotropic Materials: Under a polariscope, these materials remain dark regardless of the analyzer’s rotation because they do not affect the polarization of light.

Anisotropic Materials: These materials show changes in brightness and color as the analyzer is rotated due to double refraction, which alters the polarization state of the light passing through them.

 

2. Observing Optical Phenomena:

Interference Patterns: Anisotropic materials often show interference patterns when viewed through a polariscope, which can help identify the material’s optical properties.

Pleochroism: Some anisotropic gemstones exhibit pleochroism, where they display different colors when viewed from different angles. Polarizing filters help observe and study this phenomenon.

 

3. Detecting Strain and Inclusions:

Polarizing filters can reveal internal strain patterns and inclusions within a gemstone, providing valuable information about its quality and authenticity.

 

Practical Example

 

Consider a piece of tourmaline, an anisotropic gemstone. When viewed through a polariscope, the tourmaline will exhibit changes in brightness and color as the analyzer is rotated. This behavior indicates its anisotropic nature and helps gemologists identify the gemstone and evaluate its optical properties.

 

Conclusion

 

Polarizing filters are essential tools in gemology for producing polarized light and analyzing the optical properties of gemstones. By allowing light waves of specific orientations to pass through, these filters help gemologists identify isotropic and anisotropic materials, observe optical phenomena, and detect internal features within gemstones. Understanding the function and application of polarizing filters is crucial for accurate gemstone identification and evaluation.

 

Construction, uses and typically results of Polariscope

 

A polariscope is a critical tool in gemology used to study the optical properties of gemstones. It helps distinguish between isotropic and anisotropic materials, identify internal features, and assess optical phenomena. Here’s a detailed look at the construction, uses, and typical results of a polariscope:

 

Construction of a Polariscope

 

A standard polariscope consists of the following components:

 

1. Light Source:

Provides illumination for the gemstone. It is often a small, adjustable, and steady light, typically LED for consistency.

 

2. Polarizing Filters:

Polarizer: The first filter, placed below the stage where the gemstone is situated. It polarizes the light before it enters the gemstone.

Analyzer: The second filter, placed above the stage and can be rotated. It analyzes the light emerging from the gemstone.

 

3. Stage:

A platform where the gemstone is placed. It is usually clear to allow light to pass through the gem.

 

4. Optional Accessories:

Conoscope (or Bertrand Lens): Used to observe interference patterns in anisotropic materials, helping identify optic axes and other properties.

Dark Field Illumination: Enhances the visibility of inclusions and other internal features.

 

Uses of a Polariscope

 

1. Identifying Isotropic and Anisotropic Materials:

Isotropic Materials: These materials, like garnet and spinel, will remain dark throughout a full 360degree rotation of the analyzer, indicating no change in light behavior.

Anisotropic Materials: These materials, like quartz and tourmaline, will show changes in brightness and color as the analyzer is rotated due to double refraction.

 

2. Detecting Strain and Inclusions:

Polariscopes can reveal internal strain patterns and inclusions within a gemstone. Strain patterns may indicate the gemstone has undergone stress, while inclusions can provide clues about the gemstone’s origin and authenticity.

 

3. Observing Optical Phenomena:

Interference Figures: When used with a conoscope, a polariscope can reveal interference figures in anisotropic materials. These patterns help identify the optic axes and determine if the material is uniaxial or biaxial.

Pleochroism: Some anisotropic gemstones exhibit different colors when viewed from different angles. A polariscope helps observe and study this phenomenon.

4. Detecting Treatments:

Certain treatments and enhancements can alter a gemstone’s optical properties. A polariscope can help detect these changes.

Typical Results from a Polariscope

1. Isotropic Gemstones:

Appearance: The gemstone remains consistently dark as the analyzer is rotated.

Conclusion: The gemstone has uniform optical properties in all directions (e.g., garnet, spinel).

2. Anisotropic Gemstones:

Appearance: The gemstone shows changes in brightness and color as the analyzer is rotated.

Conclusion: The gemstone has different optical properties in different directions and exhibits double refraction (e.g., quartz, tourmaline).

3. Interference Figures (with Conoscope):

Uniaxial Figures: Display a cross or bull’seye pattern when viewed along the optic axis.

Biaxial Figures: Show two curved isogyres (black bars) that move and change shape as the stage is rotated.

4. Strain Patterns and Inclusions:

Appearance: Visible lines, bands, or irregularities within the gemstone.

Conclusion: Indicates internal stress or the presence of inclusions, providing insights into the gemstone’s formation and history.

Practical Example

Consider a piece of calcite, an anisotropic gemstone. When placed in a polariscope and viewed with the analyzer rotated, it shows changes in brightness and color due to double refraction. If examined with a conoscope, it may display a uniaxial interference figure, confirming its optical properties.

Conclusion

A polariscope is an essential tool in gemology for analyzing the optical properties of gemstones. Its construction includes a light source, polarizing filters, and a stage, with optional accessories like a conoscope. It is used to identify isotropic and anisotropic materials, detect strain and inclusions, observe optical phenomena, and detect treatments. The typical results from a polariscope help gemologists accurately identify and evaluate gemstones.

Interference figures

Interference figures are patterns observed in anisotropic materials when viewed under polarized light, typically using a polariscope equipped with a conoscope. These patterns provide valuable information about the optical properties of the material, such as the presence and orientation of optic axes, and whether the material is uniaxial or biaxial. Here’s a detailed explanation:

Formation of Interference Figures

Interference figures form due to the phenomenon of birefringence, where anisotropic materials split incident light into two rays with different velocities and refractive indices. When these rays recombine, they create constructive and destructive interference patterns that are characteristic of the material’s optical properties.

 

Types of Interference Figures

 

1. Uniaxial Interference Figures:

Description: Uniaxial crystals have a single optic axis. When viewed along this axis, the interference figure typically appears as a cross (Maltese cross) or a bull’seye pattern.

Features:

Maltese Cross: A dark cross that remains stationary as the stage is rotated. This cross is formed by the isogyres, which are the dark bands representing regions of no light transmission.

Concentric Rings: Colored rings (known as isochromes) surrounding the center, which are more pronounced near the optic axis.

Examples: Quartz, calcite, and tourmaline.

 

2. Biaxial Interference Figures:

Description: Biaxial crystals have two optic axes. The interference figure for biaxial materials appears more complex and changes shape as the stage is rotated.

Features:

Isogyres: Curved dark bands that move and change shape upon rotation. In a centered optic axis figure, the isogyres can form a shape resembling an “X” or hyperbolic curves.

Isochromes: Colored rings surrounding the isogyres, indicating different wavelengths of light experiencing constructive and destructive interference.

Examples: Topaz, orthoclase, and gypsum.

 

Observation with a Polariscope and Conoscope

 

To observe interference figures, the following setup is used:

1. Polariscope: Provides the polarized light source.
2. Conoscope (or Bertrand Lens): Used to observe the interference figures. This lens focuses the light emerging from the gemstone into a pattern that can be analyzed.

Steps to Observe Interference Figures:

1. Place the gemstone on the stage of the polariscope.
2. Ensure the polarizer is positioned below the gemstone and the analyzer is above it.
3. Use the conoscope to focus on the interference pattern created by the gemstone.
4. Rotate the analyzer and observe the changes in the interference figure.

 

Practical Applications

 

1. Determining Optic Sign:

Uniaxial Crystals: The interference figure helps determine if the crystal is optically positive or negative. This is done by examining the orientation and behavior of the isochromes and isogyres.

Biaxial Crystals: The figure provides information on the optic sign (positive or negative) based on the curvature and movement of the isogyres.

 

2. Identifying Crystal Symmetry:

Interference figures reveal the symmetry and optical orientation of the crystal, which helps in identifying the crystal system (e.g., hexagonal, tetragonal, orthorhombic).

 

3. Diagnosing Internal Stress:

Stress within a gemstone can distort interference figures, providing clues about internal strains and possible treatments or enhancements.

 

Example

 

Consider a piece of gypsum, a biaxial crystal. When observed along one of its optic axes using a polariscope and conoscope, the interference figure shows curved isogyres that move as the stage is rotated. The presence of two optic axes and the behavior of the isogyres confirm its biaxial nature.

 

Conclusion

 

Interference figures are critical in gemology for understanding the optical properties of anisotropic materials. By analyzing these patterns, gemologists can determine whether a gemstone is uniaxial or biaxial, identify its optic sign, diagnose internal stress, and gain insights into its crystal symmetry. Observing interference figures using a polariscope and conoscope is a key technique in the accurate identification and evaluation of gemstones.