ࡱ> CEB` Yjbjb $|c|cT bjjj8 $b*7XX63666666,9R;T6- 6?6???8X6? 4@"X6??P3L6 3 j^4L6 60*74\(<?(<0L6?bbD  bbGeo 407 Lecture 02 - Reflection, Refraction, Lenses, Relief, Becke Line Reading: p. 7-15 next: Ch. 3-4 Goals Finish waves Understand refraction Understand critical angle Learn Snell's Law Understand lenses (generally) Learn about relief (high, low, positive, negative) Learn about Becke Line Materials Prisms Laser thin sections of varying relief grain mount thin sections Positive and negative relief grain mounts biotite-bearing thin sections Waves Wave front Propagation direction Wave normal In non-isometric crystals, propagation `" wave normal Reflection Incoming angle = reflected angle measured relative to surface normal Refraction If light is moving into slower material, how will it bend? draw wave fronts light is slowed, but frequency stays the same Bends towards slower material (higher n) If light is moving into faster material, how will it bend? draw wave fronts Bends towards slower material (higher n)! Snell's Law gives precise angles (Eq. 1.3, p. 9): (note that theta is measured from the surface normal here as well) n1/n2 = sin (2 / sin (1 Critical Angle & Total internal reflection How big can (1 be, when going from higher n to lower? If (2 > 90, then there is no refracted light - all reflected (Total Internal Reflection) When (2 = 90, then (1 is the critical angle. Example: if light is going from ruby (corundum, n ~ 1.78) to air (n ~ 1.0), then figure out the critical angle. T.I.R. is important for gem faceting - desirable Refraction and polarization When (1 is between 0 and the critical angle, then there will be both a reflected and refracted ray Both are, in general, partially polarized reflected: in line || to surface, and normal to wave normal refracted: in line normal to vibration direction of reflected light Polarization is strongest when angle between reflected & refracted is 90. The angle between the incoming ray and the surface normal is then Brewsters angle. Lenses Work because of refraction Relief (Shagreen) Demonstrate differences in relief Why does this happen? Need to understand construction of thin section Grains are rough Light scattered at surfaces through refraction Angles of refraction depend on difference in n So, relief depends on n of medium Thin section epoxy has n=1.54 Relief can be high or low Depends on difference between nmineral & nmedium Relief can be positive or negative Depends on nmineral - nmedium Relief is different in different media Leads to immersion method for determining n Becke line [Demonstrate] Medium objective Low light (iris) Lower stage, and line moves into material with higher n Becke line is produced by two factors Reflection from vertical grain boundaries Lensing from curved grain boundaries Oblique Illumination Relies on same features as Becke line Partially obscure the light below the substage with a paper or card, and see where the shadows go. 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