Ass #7: The Path Light Travels PDF

ASS #7 : The Path that Light Travels Reflection: - Reflection: When light strikes a boundary or surface between two materials, part of it always returns to the observer - The law of reflection states that the angle at which light approaches the surface is equal to the angle at which light returns fr...

ASS #7 : The Path that Light Travels Reflection: - Reflection: When light strikes a boundary or surface between two materials, part of it always returns to the observer - The law of reflection states that the angle at which light approaches the surface is equal to the angle at which light returns from the surface. - Angle of Incidence: Light approaching a surface is called incident light, and the angle at which it hits the surface - Angle of Reflection:The angle at which reflected light returns to the observer - Normal: These two angles are measured between the normal, an imaginary line perpendicular to the surface, and the incident light ray and the reflected light ray, respectively,and the two angles are always equal. - When light strikes a surface or a boundary between different materials, for example air and a diamond, part of it is reflected, while the rest enters the diamond. - Refraction: When incident light travels from one material to another, with a change in speed and possible change in direction - Angle of Refraction: The angle between the normal and the refracted light ( like a straw in water) - The amount of refraction depends on an optical property known as optical density, which is the ability of a material to slow down light. - Refractive index (RI):measure of how the speed and direction of light change in a material compared to in air. - RI is an indication of the material’s optical density. - In gemology, the RI of air is assumed to be 1, so all gem materials have RIs higher than 1. - Materials with higher RIs are more refractive than those with lower RIs. - When light travels from a less refractive material to a more refractive material, the refracted light bends toward the normal. - When light travels from a more refractive medium (or material) to a less refractive medium, it bends away from the normal - Speed Of Light: The speed of light depends on the material it travels through. The tightly bonded carbon atoms in diamond dramatically slow incoming light. - Total Internal Reflection: -. If the angle of incidence keeps increasing, at some point the angle of refraction will reach 90 degrees before the angle of incidence does. Then the light cannot escape the material anymore, but is totally reflected back to the material. This is called total internal reflection. - The critical angle is the incident angle where the angle of refraction reaches 90 degrees and total internal reflection begins. - Materials with higher RIs have smaller critical angles, and materials with lower RIs have larger critical angles. - This angle at which light can no longer exit the diamond is 24.5 degrees—the critical angle of diamond - For total internal reflection to occur, light must hit the inner surface of a facet at an angle larger than the critical angle. - The small critical angle of diamond limits the opportunity for light to exit. -What happens to light rays outside the critical angle cone, don’t exit the stone but are reflected internally - Grease: Due to the smaller difference in RI between diamond and grease, diamond’s critical angle relative to grease is about 38 degrees. This larger critical angle and critical angle cone give light more opportunity to exit the diamond cause unnecessary light loss Dispersion: - white light separates into its spectral colors, an optical phenomenon called dispersion. This causes fire, one of diamond’s most important optical properties, Dispersion is an optical property that is characteristic for a material. - Violet light bends more than red light, so violet light has a higher RI than red light. In a diamond, the RI of violet light is 2.451, while the RI of red light is 2.407. A material’s dispersion is measured by the RI difference between the violet and red light rays in the material. Path Length: - In gemology, the total distance that a light ray travels in a gemstone is called path length. - Path length influences color appearance because of visible light absorption. -the greater distance light travels in a gemstone, the more absorption occurs. Light and Diamond s Appearance: -When light interacts with a fashioned diamond, the quality of its cut shows in its light performance attributes: brightness, fire, and scintillation. For colored diamonds, the design of the cut affects the appearance of color. Brightness: -Brightness describes the amount of light that returns to the observer’s eye. - Many light rays finally strike the crown facets within critical angles and exit the gem to the observer, contributing to the diamond’s display of brightness. - brightness is possible because of its extremely high luster—the appearance of its surface in reflected light, materials with higher RIs tend to have higher luster. - A diamonds High RI and along with its hardness allow it to achieve a high polish aka a distinctive luster described as adamantine, which means “diamond-like.” - Adamantine is the highest of all luster descriptions for transparent gemstones. Scintillation: - Scintillation has two components: sparkle and pattern.Sparkle appears as flashing spots of light. In a well-cut diamond, they’re evenly distributed and balanced in size. Pattern is the relative size, arrangement, and contrast of the bright and dark areas, as seen in the face-up position. -A diamond’s face-up pattern reveals nearly all of the cutter’s fashioning choices. Fire: - The term “fire” describes a diamond’s display of dispersion. Fire refers to the rainbow colors visible when you move a well-cut diamond under an overhead spotlight. - Fire results from the arrangement of a diamond’s facets and the angles between them. -Four factors influence the fire seen in the face-up view of a diamond: First the angle at which light enters the diamond, greater the angle, the greater the refraction, -Second the number of times a light ray interacts with the diamond’s internal facets, -Third angle of the light rays as they exit the diamond affects fire, smaller the exit angle, the larger the angle of refraction spread farther apart, creating greater fire -Lighting can also affect fire light striking from all angles emphasizes a diamond’s brightness but suppresses its fire. - A mix of fluorescent and incandescent lighting creates a balance between brightness and fire. Color Appearance: - Through cut design, cutters can either shorten or lengthen the path that light travels in a diamond. -A shortened path length results in less absorption, which gives the diamond a lighter color. A longer path length increases absorption and gives the diamond a deeper color. Cut Grade: - GIA Cutting Grading system was developed an advanced light ray–tracing computer program to understand how light behaves within a round brilliant diamond helping them determine which proportion combinations create light performance attractive to most observers, observation assessed brightness, fire, and cut appearance for diamonds of many different proportions. - Result, each cut grade allows a variety of proportion combinations and appearances - Researchers discovered that personal and regional preferences in appearance were an inherent part of a functional grading system. This emphasizes the fact that there can be various equally attractive appearances within a grade category. GIA Cut Grading System: -The GIA Cut Grading System applies to round brilliant diamonds in all clarities across the D-to-Z color range. -The system assigns one of five cut grades: Excellent (Ex), Very Good (VG), Good (G), Fair (F), and Poor (P). - Design quality and craftsmanship should be considered when grading a round brilliant diamond. -Design refers to a fashioned diamond’s proportions and durability, which are determined by the cutters during the cutting process. -Craftsmanship is evidenced by the gem’s polish and symmetry. -Polish is the overall condition of the facet surfaces of a finished diamond. - Symmetry describes the exactness of a finished gem’s shape and the placement of its facets. Grading Environment: - An important first step is to make sure the lighting environment allows for consistent and repeatable results. - Light impacts diamond so dramatically that the same diamond can look quite different under different types of lighting and in different positions. - a standardized viewing environment and neutral background are essential for consistent grading results. - Daylight-equivalent diffused fluorescent light for judging brightness and face-up pattern and an array of light-emitting diodes (LEDs) for judging fire. - - Grading Process: - fluorescent light to assess the diamond’s brightness and pattern. Depending on how “lively” the stone looks, it gets a brightness rating of Excellent to Poor. - switching to spot lighting, the grader evaluates the diamond’s fire. Based on the flashes of fire that are visible, the diamond’s fire is rated Excellent to Poor.