Sept 4 2003 Mineralogy 231 Recitation 1 Introduction to Optics readings tomorrow in class uniaxial indicatrix and optic sign Chap 7 Nesse p 126 133 one class lecture will be shifted to the recitation section in a few weeks today s lecture optics of isotropic substances next Thursday anisotropic substances nature of light part of electromagnetic spectrum wavelength of visible light ranges from ca 400 violet to 700 red nm 10 nm 1 invisible to eye infrared rays have longer wavelengths ultraviolet have shorter wavelengths dual properties of light photons both in terms of particles and waves for ordinary optical observations consideration of wave properties alone will suffice light produced by intense temperatures emission of radiation in visible range white light contains all visible wavelengths as can be seen with a prism or rainbow petrographic microscopes usually employ a blue filter to adjust color and reduce yellow hues Fig 6 6 color blind individuals still possible to do optics but other cues have to be used properties of light as a migrating wave different waves vibrate in all directions perpendicular to direction of propagation ray path each wave has given amplitude vibration direction both electric and magnetic vectors can be resolved into any two mutually perpendicular vectors polarizer interacts with light to permit only those components vibrating in a single plane to pass only vectors in one plane for each wave of the incoming light continue through compound if second polarizer is placed at right angles to first no light passes through one of the two mutually perpendicular vectors will pass through polarizer for each incoming wave interactions of light and matter velocity of light decreased by presence of matter interaction of waves w electrons slows them down index of refraction n refractive index R I or simply RI used interchangeably c v where c is the velocity of light in a vacuum v is its velocity in a medium n 1 in a vacuum n 1 in matter e g 1 0003 air 1 33 for water 1 54 for NaCl 1 55 for epoxy n varies between 1 45 and 2 0 for most minerals 2 4 for diamond and sphalerite ZnS so what is the speed of light in diamond index of refraction of a compound generally well correlated with density light has lower speed in material of higher density and RI crystalline compounds can be separated into isotropic and anisotropic v is constant in all directions in an isotropic compound v is variable in different directions in an anisotropic compound the much more common case reflection and refraction consider a flat surface between two materials with a different n Fig 7 6 typical examples air liquid liquid solid when light strikes the boundary obliquely both reflection and refraction may occur reflection angles of incidence and reflection are equal Fig 7 6a refraction light path bent by passage from one medium to the next reflected light is partially polarized horizontally refracted light vertically Bloss Fig 2 1 what everyday observation shows horizontal polarization of reflected light hint use of a fashion item Snell s law For refraction sin 1n1 sin 2n2 sin 1 sin 2 n2 n1 refraction occurs because wave front is delayed by entry into medium with higher n analogous to driving a car off the road obliquely Fig 7 6b critical angle light is successively more bent toward horizontal upon passage into medium of lower n below some angle no light is refracted and all is reflected the angle below which all is reflected is the critical angle where does the light go at exactly the critical angle so there is a blind spot whence objects cannot be seen fly fishermen are well aware of this measurement of critical angle can be used to obtain the RI of a polished surface commonly employed for gems to measure RI without crushing sample also can be used to measure RI of liquids with known RI of a solid usually lead glass why Pb RI measurement of isotropic solid velocity of light is not conveniently measured directly use a microscope successively suspend grains of compound in liquids of known RI grain nearly disappears from view if RI matches relief qualitative measure of RI grain stands out if its RI differs from that of medium negative vs positive relief Becke line illumination around edge of grain light concentrated outward if RI is less than the immersion medium light concentrated inward if RI is greater than the immersion medium this effect is not caused by a lens shape as is shown also works for vertical edges Fig 7 1 Figs 7 54 7 55 Figs 7 56 7 59 Fig 7 57 Fig 7 58 practical aspects 1 use parallel light source medium magnification medium illumination Kohler arrangement 2 to see Becke line gently raise objective in reality one is lowering the stage counterclockwise motion of right focusing knob clockwise motion of left knob 3 view grain under uncrossed polars as stage is lowered 4 watch for movement of faint light line on margin of grain Fig 7 59