LIGHT PROPERTIES Flashcards
What do optical properties depend on?
-manner which visible light is transmitted through crystal -crystal structure -crystal symmetry -chemical composition
in the C = νλ equation, what does it mean and tell me about it
frequency of vibration (v) is constant, so if velocity (C) is reduced then the wavelength (λ) is decreased
tell me about refractive index (n)
-the ratio of the speed of light in a vacuum, C, to the speed of light in a material through which it passes, Cm. n = C/Cm -always 1.0< since Cm is never greater than C. Cm depends on the density of the material (Cm decreases with increasing density) -higher density means higher RI -The refractive index of any material depends on the wavelength of light because different wavelengths are interfered with to different extents by the atoms that make up the material. In general refractive index varies linearly with wavelength.
Materials can be divided into 2 classes based on how the velocity of light of a particular wavelength varies in the material. What are they?
-Materials whose refractive index not depend on the direction that the light travels are called isotropic materials. In these materials the velocity of light does not depend on the direction that the light travels. Isotropic materials have a single, constant refractive index for each wavelength. Minerals that crystallize in the isometric system, by virtue of their symmetry, are isotropic. Similarly, glass, gases, most liquids and amorphous solids are isotropic. -Materials whose refractive index does depend on the direction that the light travels are called anisotropic materials. These types of materials will have a range of refractive indices between two extreme values for each wavelength. Anisotropic materials can be further divided into two subclasses, although the reasoning behind these subdivisions will become clear in a later lecture.
What are the two classes of anisotropic minerals?
-Minerals that crystallize in the tetragonal and hexagonal crystal systems (as well as some plastics) are uniaxial and are characterized by 2 extreme refractive indices for each wavelength. -Minerals that crystallize in the triclinic, monoclinic, and orthorhombic crystal systems are biaxial and are characterized by 3 refractive indices, one of which is intermediate between the other two.
tell me about reflection and refraction
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tell me about the critical angle for total internal reflection
tell me about dispersion of light
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tell me about absorption of light
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the image shows the sunlight passing through the atmosphere. this is the absorption curve
tell me about polarization of light
normal light vibes perpendicular to path of propogation. if it is constrained to vibrate on one plane, it is plane polarized light. (direction it travels is called vibration direction, which is perp to direction…TIM)
what are two common ways light can be polarized?
- reflection off a nonmetallic surface (class/paint). An unpolarized beam of light, vibrating in all directions perpendicular to its path strikes such a surface and is reflected. The reflected beam will be polarized with vibration directions parallel to the reflecting surface (perpendicular to the page as indicated by the open circles on the ray path). If some of this light also enters the material and is refracted at an angle 90o to the path of the reflected ray, it too will become partially polarized, with vibration directions again perpendicular to the path of the refracted ray, but in the plane perpendicular to the direction of vibration in the reflected ray (the plane of the paper, as shown in the drawing).
- can also happen by passing light through a substance that absorbs the light vibrating in all direction but one. anisotropic minerals have this property in specific directions (priviledged directions). Crystals were used to produce polarized light in microscopes built before about 1950. The device used to make polarized light in modern microscopes is a Polaroid, a trade name for a plastic film made by the Polaroid Corporation. A Polaroid consists of long-chain organic molecules that are aligned in one direction an placed in a plastic sheet. They are placed close enough to form a closely spaced linear grid, that allows the passage of light vibrating only in the same direction as the grid. Light vibrating in all other directions is absorbed. Such a device is also called a polarizer. If a beam on non-polarized light encounters a polarizer, only light vibrating parallel to the polarizing direction of the polarizer will be allowed to pass. The light coming out on the other side will then be plane polarized, and will be vibrating parallel to the polarizing direction of the polarizer. If another polarizer with its polarization direction oriented perpendicular to the first polarizer is placed in front of the beam of now polarized light, then no light will penetrate the second polarizer. In this case we say that the light has been extinguished.
tell me how polarized sunglasses work.
incoming solar radiation hits a reflective surface and becomes polarized (travelling // to reflective surface. polarized glasses contain polarizers that are perpendicular to the polarized reflected light. thus all the horizontal polarized light from the reflected surfaces will be blocked out
tell me how a polarized microscope works
- 2 polarizers that are perpendicular to each other
- Light from a light source located below the tube and stage of the microscope is initially unpolarized. This light first passes through the lower polarizer (usually just called the polarizer), where it becomes polarized such that the light is vibrating from the users right to left. These directions are referred to as East (right and West (left). The light then passes through a hole in the rotatable stage of the microscope and enters the lower lens, called the objective lens.
- 2nd polarizer is the analyzer that can be rotated or pushed so that in can be in the light path (inserted position) or not in the light path (analyzer out position). The analyzer has a polarization direction exactly perpendicular to that of the lower polarizer These directions are usually referred to as North - South. If the analyzer is in, then the plane polarized light coming from the lower polarizer will be blocked, and no light will be transmitted though the ocular lens above.
- If the analyzer is out, so that it is not in the light path, then the polarized light will be transmitted through the ocular lens.
tell me about the isotropic indicatrix
- The concept of the optical indicatrix is important as a visual means of looking at the way refractive index varies with direction in a substance.
- indicatrix for isotropic materials isnt that important because the RI doesnt vary with direction
- The optical indicatrix is simply a three dimensional object constructed by drawing vectors of length proportional to the refractive index for light vibrating parallel to the vector direction from a central point. The ends of all of the vectors are then connected to form the indicatrix. For isotropic minerals the indicatrix is a sphere as can be seen here. The indicatrix can be placed anywhere within or on a crystal so long as the crystallographic directions in the indicatrix are moved parallel to themselves. Again, for the isotropic indicatrix, this is fairly trivial since the refractive indices do not correspond to crystallographic directions and the refractive indices are the same in all directions, but the usefulness of the indicatrix concept will become much more clear when we look at anisotropic substances.
tell me about isotropic materials and polarized light
- viewing the mineral from the ocular lens will be able to see the grain clearly
- if the grain selectively absorbs light of certain wavelengths we will see absorption color
- inserting the analyzer will cause the mineral to go extinct (even upon rotation) because isotropic materials do not change the polarization direction
****If the grain remains extinct throughout a 360o rotation of the stage, then the mineral or substance on the microscope stage is probably isotropic
explain the Becke Line Method
- becke line will aways go closer to the material with the higher RI
- how to perform the test is outlined below:
1. focus the microscope sharply as possible (also use the iris diaphram)
2. Then using the fine focus dial adjustment the microscope stage is lowered (or the objective lens is raised) slightly out of focus. During this increase in focal distance one observes a moving bright Becke line. If the Becke line moves inward, the refractive index of the grain is greater than the refractive index of the oil.
NOTE: ****we have to INCREASE the distance between the grain and objective lens to perform the test**** therefore move the stage AWAY from you
-Recall from above that we said that refractive indices for grains (and also oils) are reported for a specific wavelength of light. That wavelength is 589 nm, which corresponds to yellow. Since we are using white light as an illuminator for our grain, the Becke line will be different for different wavelengths or colors of light.
clearly define uniaxial anisotropic minerals clearly
UNIAXIAL
- hex, trig, tetra-in these classes because only has ONE OPTIC AXIS
- they have TWO RI
- Light travelling // to c axis travels undisturbed through crystal (SO THE INCIDENT RAY IS NOT SEPARATED INTO 2 RAYS). the incident ray travels as ordinary ray
- However, if the light travels through the mineral in any direction other than parallel to the c-axis, the light is separated into two rays, the ordinary ray and the extraordinary ray. The ordinary ray vibrates in the basal plane perpendicular to the c-axis, while the extraordinary ray will vibrate in a plane perpendicular to the ordinary ray vibration, which contains the c-axis.
- optically + minerals=prolate
- optically - minerals=oblate
- Uniaxial minerals are characterized by a ‘cross-shaped’ interference figure (Figure 5) that is produced by inserting the condenser lens and the Bertrand lens with the microscope set to its highest power objective lens (60x).
- ***ALL ANISO CAN DISPLAY DBL REFRACTION WHICH SEPARATES EACH INCIDENT RAY INTO TWO RAYS (ORINARY RAY AND EXTRODINARY RAY)
- ORINARY RAY-vibe direction perpendicular to propogation direction
- EXTRODINARY RAY-vibe direction NOT PERPENDICULAR to propogation direction
how do we determine if a uniaxial mineral is optically +/-?
- use interference figures (insert bertrand lens and condenser lens with highest power setting)
- The vibration directions of the light in the dark areas is parallel to the vibration directions of the polarizer and analyzer. With no light passing through in these locations a dark cross results. Each segment of the cross is called an isogyre, and the place where they intersect the melatope (which represents the c-axis or optic axis).
NOTE for some minerals, the optic sign is diagnostic, for others two optic signs can occur.
