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Optical Mineralogy

Optical Mineralogy - 1 Optical Mineralogy The...

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1 Optical Mineralogy The electromagnetic spectrum (Fig. 1) Nature of light: Velocity (V) Wavelength ( λ ), amplitude (A), and frequency ( ν ) (Fig. 2a) V = ν . λ Energy: E = h ν , where “h” is Plank’s constant (6.625 . 10 -34 J second). Intensity of light is proportional to A 2 . Frequency: remains constant regardless of the medium that light travels through. Therefore, when the medium through which light is passing changes, the velocity and the wavelength change (Fig. 2b). Light vibrates in all directions within a plane perpendicular to the direction of propagation. Light vibrating in one direction only (plane polarized light) can be obtained with the help of a polarizer (Fig. 3). Wave interference: Retardation: = i. λ if “i” is a whole number, the waves are considered in phase if “i” is a fraction, the waves are considered out of phase (Fig. 4). Constructive (Fig. 4a & b) vs. destructive interference (Fig. 4c). Refraction Refractive index of a substance “n” is inversely proportional to the velocity of light passing through it. Snell’s law: (Fig. 5) sin i/sin r = n 2 /n 1 = v 1 /v 2 Refractive index of balsam = 1.537 Critical angle : Is the angle of incidence that yields an angle of refraction = 90° (Fig. 6). Total internal reflection : occurs for all angles of incidence > the critical angle. Applications: 1- The Optical Refractometer (Figs. 7a & b). 2-The Becke line test (Fig. 8): how it makes use of total internal reflection (Fig. 9) and the lens effect (Fig. 10) (cf. text page 153). Light absorption and the Origin of Color Check your notes on the physical properties of minerals! Dispersion of Light: Violet wavelengths are more strongly refracted than red ones (i.e. have a higher angle of refraction). This is known as normal dispersion.
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2 Abnormal dispersion results when the mineral absorbs a portion of light (some wavelength). This happens only in colored minerals.
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Optical Mineralogy - 1 Optical Mineralogy The...

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