This Chapter is planned, organized and described by Dr. T K Bansal for visually impaired and blind students. All efforts have been made to make the chapter fully accessible by providing headings, describing figures and tables. The Chapter has been so designed that the students should enjoy studying this chapter and should move forward without any hurdles. The chapter has been thoroughly checked for any mistakes, still if you find any mistakes or have any suggestions to make it better, please write to us at ‘ blind2Visionary@gmail.com ‘ or leave your comments at ‘www.blind2visionary.org’. Your efforts will be highly appreciated.

Table of symbols used in this chapter