312 1933 E. SCHRODINGER Fig. 6 dent light). All of youare undoubtedly familiar with the so-called"motes of dust" in a light beam falling into a dark room. Fine blades of grass and spiders webs on the crest of a hill with the sun behind it, or the errant locks of hair of a man standing with the sun behind often light up mysteriously by diffracted light, and the visibility of smoke and mist is based on it. It comes not really from the body itself, but from its immediate surroundings, an area in which it causes considerable interference with the incident wave fronts. It is interesting, and important for what follows, to observe that the area of interference always and in every direction has at least the extent of one or a few wavelengths, no matter how small the disturbing particle may be. Once again, therefore, we observe a close relationship between the phe. nomenon of diffraction and wavelength. This is perhaps best illustrated by reference to another wave process, i.e. sound. Because of the much greater wavelength, which is of the order of centimetres and metres, shadow for- mation recedes in the case of sound, and diffraction plays a major, and prac tically important, part: we can easily hear a man calling from behind a high wall or around the corner of a solid house, even if we cannot see him Let us return from optics to mechanics and explore the analogy to its fullest extent. In optics the old system of mechanics corresponds to intelle312 1933 E. SCHRÖDINGER Fig. 6. dent light). All of youare undoubtedly familiar with the so-called "motes of dust" in a light beam falling into a dark room. Fine blades of grass and spiders’ webs on the crest of a hill with the sun behind it, or the errant locks of hair of a man standing with the sun behind often light up mysteriously by diffracted light, and the visibility of smoke and mist is based on it. It comes not really from the body itself, but from its immediate surroundings, an area in which it causes considerable interference with the incident wave fronts. It is interesting, and important for what follows, to observe that the area of interference always and in every direction has at least the extent of one or a few wavelengths, no matter how small the disturbing particle may be. Once again, therefore, we observe a close relationship between the phe￾nomenon of diffraction and wavelength. This is perhaps best illustrated by reference to another wave process, i.e. sound. Because of the much greater wavelength, which is of the order of centimetres and metres, shadow for￾mation recedes in the case of sound, and diffraction plays a major, and prac￾tically important, part: we can easily hear a man calling from behind a high wall or around the corner of a solid house, even if we cannot see him. Let us return from optics to mechanics and explore the analogy to its fullest extent. In optics the old system of mechanics corresponds to intellec-