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New, expanded 2017 edition - more than 200 000 readers.This physics text presents the foundations of quantum physics in a simple way, with few formulae, but using many puzzles and observations taken from everyday life. The volume shows that colours, atoms and living beings only exist because nature features a smallest change, the quantum of action, also called Planck's constant. This quantum of change leads to wave functions, probabilities, tunnelling and colours.This fourth volume (264 pages) of the Motion Mountain physics text provides a simple, modern and surprising introduction to quantum effects in matter and light, with many figures and photographs.The text, printed in black and white, is written in an easy style, uses little mathematics, and is accessible to undergraduates, teachers and anybody who is interested in the precise description of nature. Enjoy the reading!