Earth Optics Video 3: Fast & Slow Ray

In this video, I introduce the concept of light propogating through minerals as a faster and slower ray. Examples use the accessory plate to determine orientation of a faster and slower ray.
For supplemental questions and resources, please check out my webpage at www.earthopticsmineralogy.com
Accessory Plate Image:
micro.magnet.fs...
Michel-Levy Chart:
University of Liverpool. D Flinn G Newell

[@19midnightsun87]

Really cool how to see that the first order colors even reverse, because the fast ray is now slower than the slow ray.

[@obliquecolumn834]

My comments are similar to Erik's below: I understand that a grain can have 'fast length' or 'slow length' orientation on the stage but how can a mineral, Ex lawsonite, be described as being 'length fast' as an attribute.
Also with respect to in the grain sample of your video where we were to determine if it was fast or slow, with respect to the Michel Levy chart and looking from the starting position of magenta: I couldn't see how the light aqua colour [at orientation +45 degrees] was deemed 'length fast' and that the white [at orientation -45 degrees] was 'length slow'.
Thank-you for your videos. They are the best mineralogy videos on "UA-cam

[@EarthOpticsVideos]

Hi! Thanks for your feedback :D
The orientation is key. A section (or slice) of a mineral grain can be "length fast" over a different section of the same mineral. Along different orientations of a mineral grain, the distance between the fast and slow ray will change. Many minerals, like lawsonite, can be "sliced" into different sections - for example, a rectangular prism with a long c-axis can be sliced parallel to the c-axis and have the shape of a rectangle with a long aspect ratio OR it can be sliced perpendicular to the c-axis and have the shape of a square. These sections have different orientations and will widen (or close) the gap (or retardation) between the fast and the slow ray of light passing through your sample. If you see "length fast" or "length slow" in a textbook, it is likely referring to the position or section of the mineral grain that preferentially widens the gap between the fast and slow ray of light ("length fast") or closes the gap ("length slow"). I go into characterizing minerals along their crystallographic axes in later videos, if you are interested (see uniaxial flash figure and biaxial flash figure).
To answer your second question - from magenta, the light aqua color is a higher order interference color (second order). Since the retardation increases (as represented by the higher order color observed), we can say that the fast ray is propagating distinguish-ably faster than the slow ray, due to the orientation of the grain (the gap between the fast and slow ray widens). From magenta, white is a lower order interference color (to the left on the chart). The retardation decreases here because the orientation of the mineral grain closes the gap between the fast and slow ray. I'd be happy to try explaining another way if this doesn't make sense.
Happy mineralogy-ing! - Sam

[@jasonkepler6527]

Awesome work! Really appreciate it!

[@EarthOpticsVideos]

thanks :D

[@aintfalco7968]

I always find your videos helpful, but I'm still struggling with fast vs slow a little bit. I think some of the terminology I'm seeing from other sources is confusing me; can a mineral be described as length fast or length slow? It seems to me that there is a fast ray and a slow ray and the orientation of the grain affects the maximum birefringence seen in the mineral, so what would it mean for a mineral to be "length fast" or "length slow?"

[@EarthOpticsVideos]

Erik - Short answer to your first question is no - a mineral cannot be described as "length fast" or "length slow." As plane polarized light hits the thin section, it splits into a faster and slower ray. A mineral grain position can affect the distance (retardation) between the fast and slow ray of light as they pass through the grain, and you are correct - this is dependent on the grain's orientation with respect to its crystallographic axes. Along different orientations of a mineral grain, this distance between the fast and slow ray will change. If you see "length fast" or "length slow" in a textbook, it is likely referring to the position or section of the mineral grain that preferentially widens the gap between the fast and slow ray of light ("length fast") or closes the gap ("length slow"). Wish I saw this comment earlier - I hope it's still of help. Let me know if this needs more explaining (or different wording) Happy mineralogy-ing! - Sam

[@photon4269]

Fantastic explanations

[@naiky11]

very helpful. thank you.

[@Invisible__Woman]

You make me interested in the earth more than I can say.

[@EarthOpticsVideos]

thank you so much!

[@Luna-hr6bm]

You're such a great help! T^T Thank you!

[@akashsinghrajput2943]

Thanks a lot for videos..