Avengers at Savoy

The recently released "Avengers" movie will be screened at Savoy Wellawatte from the 11th of May 2012. This showdown of Marvel superheroes, is in my opinion one of the three most anticipated movies of 2012, the other two being "The Dark Knight Rises" and "Prometheus". It would be great if it was screened in 3D at MC superior 3D, however watching it in 2D at Savoy is better than not watching at all.. ;)  All previous Marvel superhero movies, Iron Man, Thor, Hulk and Captain America led to the events of this movie. The post credit scenes in those movies indicated it. After a long wait now that the movie will finally be shown in Sri Lanka, Marvel fans will be counting their fingers till the 11th of May.
Tickets can be booked at http://www.eapmovies.com/movie/the-avenger-english-movie/

UPDATE : Great news! Avengers will be screened in 3D at MC superior as well. Tickets can be purchased at http://www.ticketslk.com/

Optical properties of Minerals under Cross Polarized Light (CPL)

The CPL arrangement of the microscope is withe analyzer in the "In" position. The optical properties that can be observed under this arrangement are as follows,

1.  Isotropic and Anisotropic Minerals
All minerals are either Isotropic or Anisotropic. Isotropic minerals are the minerals that do not show any colour under the CPL arrangement whatsoever. These minerals appear dark. Isotropic minerals are the minerals that are very symmetric and usually belong to the cubic crystal system. Examples include Garnet and Spinel. (Garnet having pink as its optic colour turns dark when the analyzer is in. This property is used to identify garnet).
Anisotropic minerals are the minerals that show colours under the CPL arrangement. These colours are known as interference Colours. Anisotropic minerals belong to crystal systems having lower crystal symmetry. (Systems other than Cubic)

2.  Extinction Position and Angle.
Extinction position is a position reached when the microscope stage is rotated under the CPL arrangement and no colours are seen (dark positions). All Anisotropic minerals have extinction positions. All minerals show 4 extinction positions in a single rotation (90 degrees apart). At he extinction position the vibration directions of the mineral are parallel to the planes of polarization of the polarizer and the analyzer. Therefore no light reaches the eyepiece thereby giving the dark extinction position.

The extinction angle is the angle between one of the cross-hairs and the lines of cleavage of the mineral at the extinction position.

3.  Interference Colours
The colours that anisotropic minerals show under the CPL arrangement are known as Interference Colours. The maximum intensity of these colours are seen in between two extinction positions. (at 45 degree positions).

4.  Birefringence
Birefringence is the difference between the maximum and minimum Refractive index value shown by a mineral. The interference colours shown by a mineral depend on its birefringence and therefore birefringence is very useful in mineral identification.

5.  Optic Sign
Double refraction of all anisotropic minerals results in them having two refractive indices RIO and RIE. According to these values, The Optic sign is determined as follows.
RIO  >  RIE.  -  Negative (-)
RIO  <  RIE.  -  Positive (+)

6.  Interference Figures
Interference figures are important optical properties that are used in the identification of minerals, as well as to determine other properties of minerals such as,
-  Optic Sign
-  Whether a mineral is Uniaxial or Biaxial
-  2V angle of a mineral (2V angle is the angle between the two optic axes of a mineral)
Interference figures are observed under the Conoscopic Arrangement of the microscope. The thin sections have to be specifically prepared in order to observe Interference figures.

The Becke Line Method

Becke line is a band of light seen along the mineral grain boundary under the PPL arrangement when the microscope is slightly out of focus. Depending on the way the microscope is focused, the Becke line may lie within or outside the grain boundary.

The Becke line method is used to determine the relative refractive indices of two minerals. This is performed by lowering the microscope stage or increasing the focal distance (increasing the distance between the section and the objective). When this is done the Becke line will appear to move towards the material with the higher refractive index. This method is used to compare the refractive indices of two minerals or to compare the mineral with a medium with known refractive index such as Canada Balsam or Epoxy glue.

Conoscopic arrangement of the Petrological Microscope

Observing interference figures of minerals is done under the Conoscopic view of the petrological microscope. The conoscopic arrangement is set up as follows,

1.  The medium or low power objective is selected and selected grain is brought to the centre of the cross-hairs.

2.  The light intensity is increased by using the concave mirror or the intensity controller of the electric bulb.

3.  The objective is now changed to high power and brought very close to the the thin section (almost touching it).

4.  The analyzer is put to the "IN" position.

5.  The Bertrand Lens is also put to the "IN" position. (This will make the viewing scope small)

Optic Axes of a Mineral

When double refraction is observed using a calcite crystal and when its rotated and tilted at one point the two images (the Ordinary image and the Extraordinary image) coincide and appears as one image. This direction in which no double refraction can be observed is the Optic Axis direction. While some minerals have only a single optical axis while others have two optic axes. Minerals are thus classified as follows
1.  Uniaxial Minerals - one optic axis
2.  Biaxial Minerals - two optic axes
Minerals that belong to the tetragonal and hexagonal crystal systems are uniaxial while minerals that belong to monoclinic, triclinic and orthorhombic crystal systems are biaxial.
eg.  quartz, calcite - hexagonal - uniaxial   pyroxene - orthorhombic and monoclinic - biaxial
 

Double Refraction in Minerals

Double refraction is the splitting of a single ray of light into two refracted rays when passing through an anisotropic mineral. All anisotropic minerals exhibit double refraction.
Double refraction can be practically observed by placing a transparent Calcite (Iceland spar) crystal on a sheet of paper with a black dot on it. When the dot is viewed through the crystal, two dots are seen. Also when the crystal is either rotated or tilted one of those images moves while the other remains stationary. This implies that the mineral has two refractive indices, one constant and one that varies with direction. The fixed image is called the "Ordinary image" and the moving image is called the "Extraordinary image". Similarly the rays that produce the images are also named "O-ray" and "E-ray". The Refractive indices are also identified as RIO and RIE. The relative magnitudes of RIO and RIE determine the Optic Sign of the mineral.

calcite crystal showing double refraction. image from : wikipedia

Isotropic and Anisotropic Minerals under the CPL arrangement

In the CPL arrangement of the petrological microscope, Isotropic minerals do not show any colour while anisotropic minerals show interference colours.
Isotropic minerals have high symmetry and therefore as shown in the diagram below does not alter the plane of vibartions of light that comes from the polarizer. When this light reaches the analyzer, it cannot pass through because the plane of polarization of the analyzer is perpendicular to that of the polarizer. Therefore no light moves through to the eyepiece resulting in the mineral becoming dark.

Anisotropic minerals have low symmetry and therefore alters the plane of vibration of light that passes through it. Therefore when this "altered" light reaches the analyzer, the light that has the same vibration direction as the analyzer passes through to the eyepiece. Therefore colours are seen in this case.