Within twelve years of publication of its first edition, the fourth edition of the book is in your hands. This indicates the popularity and success of previous three editions of this book. It becomes possible because of its readability, clear text and large number of simple diagrams. Due to the time constraint (students hardly get 8 to 9 months for active studies in first year) both teacher and students are under pressure to complete the syllabus. In the present edition, the text has been revised with the view to reduce the length of the book. This was achieved by dropping the unnecessary details that are irrelevant to the need of undergraduate students. In this edition, some complicated aspects are further explained for the conceptual understanding of the subject. As in earlier editions, the language has been kept simple. Few new illustrations and photographs have been added and some of the earlier ones have been improved. I gratefully acknowledge the help and cooperation received from Mr. Pawaninder P. Vij and Mr. Anupam Vij of Peepee Publishers and Distributors (P) Ltd. and their entire team during the preparation of fourth edition. I would highly appreciate and welcome suggestions from teachers and students to improve the book. Their suggestions would be carefully considered for next edition.