Earth Optics Video 5: Uniaxial Flash Figure

[@SHUBHAMMISHRA-sb1we]

Would me more than happy to see more videos in the series on other optical properties..really brief n crystal clear explanation

[@Zloner]

2:50 I think there is a mistake regarding the color interpretation while accessory plate is inserted. Second order blue means higher retardation, meaning a higher refractive index and slow ray than the second order orange showing the fast ray. The gypsum plates I work with are inserted from the SE and the gypsum's epsilon is in the NE direction and show similar optical traits as yours.

[@EarthOpticsVideos]

We interpret the yellowish-orange color in this video as a first-order retardation. We make this distinction based on the vibrancy of the orange... at higher orders of retardation, we would expect this orange color to be muted and pale. First order orange is lower than second order blue, leading to our interpretation that principal axis omega is ~parallel to the direction of the accessory plate when we see that second order blue. This interpretation is only valid for optically positive, uniaxial minerals. Hope that helps - happy microscop-ing! -Sam

[@UNTHESUNTHESUNTHES]

I still don't understand what did you find out exactly with the flash figure and why

[@EarthOpticsVideos]

It's basically a check. If you see a flash figure, you know the orientation of your mineral grain, with respect to its principal axes. I find it helpful to imagine the 3D ellipsoid. If a flash figure is present, the mineral grain you are examining is oriented such that the 2D 'slice' of the ellipsoid you are examining is perpendicular to one omega axis and is parallel to axes epsilon and omega. If you don't see a flash figure, you can imagine that the slice of the mineral exposed on your thin section is closer to being perpendicular to epsilon and parallel to an omega-omega 'slice'. This is useful for certain minerals that display different pleochroism, birefringence, or indices of refraction with respect to their crystallographic orientation.

[@mbisetakobana]

okay i got lost towards the end

[@EarthOpticsVideos]

Towards the end, I refer to skills introduced in the fast and slow ray video, which I have now linked in the video. Happy to answer any further questions. :D -Sam