Beauty, Form and Function: An Exploration of Symmetry Asset No. 5 Lecture I-4 Point Symmetry PART I Concepts in Symmetry
Objectives By the end of this lecture, you will be able to: formally examine reflection and rotation operations in a variety flowers assign the mathematical symbols of 2-fold (diad), 3-fold (triad), 4-fold (tetrad) and 6-fold (hexad) rotation points compose the point symmetry symbols of flowers
Rotational Symmetry in Flowers rotation angle = 360 o /2 = 180 o rotation angle = 360 o /4 = 90 o rotation angle = 360 o /3 = 120 o rotation angle = 360 o /6 = 60 o rotation angle = 360 o /5 = 72 o 2 mirror lines (both unique) 3 mirror lines (1 is unique) 4 mirror lines (2 are unique) 6 mirror lines (2 are unique) 5 mirror lines (1 is unique) But fold rotation is never used in crystallography. Why is this so? 2mm (both unique) 4mm 3m (not 3mmm) 6mm
Summary Point symmetry involves only reflection and rotation In assigning point symmetry it is necessary differentiate reflection lines The point symmetry symbol always has the sequence ‘rotation - mirror type 1 - mirror type 2’ (e.g. 6mm). The symbol is shortened if no rotation is present, or all the mirror lines are identical The assignment of point symmetry is independent of the object
Bibliographies tbn3.gstatic.com/images?q=tbn:ANd9GcS77WdmjC6W4MB7cOk9 avTNwZcQIzcd9PVOtOh3NhqukVYWJWGp Date retrieved: 9 Jan not_closeup_2005_01.jpg Date retrieved: 13 Jan _Milii_flowers.jpg
Bibliographies tbn1.gstatic.com/images?q=tbn:ANd9GcQ1e3H4htdWKLvXBYuaq H_CJB-rBDlq8d_CCXRgFlmww4CeqA0M tbn0.gstatic.com/images?q=tbn:ANd9GcSYf91NGmSsbpz13MFZ cHDaarhamoGHneh1DzhkHpiIgrUVb-sfQQ Date retrieved: 9 Jan 2014