Newtons Lichtbeugung
1730
Kurzinhalt
Isaac Newton nimmt Stellung zur Lichtwellen-Vermutung des Italieners Francesco Maria Grimaldi. Das Zitat aus Newton Buch „Opticks“ beschreibt das Phänomen der Beugung (diffraction): Der Schatten eines Objekts sei größer als nach der Logik der Strahlenoptik möglich. Kurz erwähnt werden auch Streifen im Sinne eines Beugungsmusters am Schattenrand. Es bleibt ein ungeklärter Widerspruch am Ende. Das steht hier im englischen Originaltext.
Original-Zitat
Observations concerning the Inflexions of the Rays of Light, and the Colours made thereby.
Grimaldo has inform'd us, that if a beam of the Sun's Light be let into a dark Room through a very small hole, the Shadows of things in this Light will be larger than they ought to be if the Rays went on by the Bodies in straight Lines, and that these Shadows have three parallel Fringes, Bands or Ranks of colour'd Light adjacent to them. But if the Hole be enlarged the Fringes grow broad and run into one another, so that they cannot be distinguish'd. These broad Shadows and Fringes have been reckon'd by some to proceed from the ordinary refraction of the Air, but without due examination of the Matter. For the circumstances of the Phænomenon, so far as I have observed them, are as follows. [Pg 318]
Obs. 1. I made in a piece of Lead a small Hole with a Pin, whose breadth was the 42d part of an Inch. For 21 of those Pins laid together took up the breadth of half an Inch. Through this Hole I let into my darken'd Chamber a beam of the Sun's Light, and found that the Shadows of Hairs, Thred, Pins, Straws, and such like slender Substances placed in this beam of Light, were considerably broader than they ought to be, if the Rays of Light passed on by these Bodies in right Lines. And particularly a Hair of a Man's Head, whose breadth was but the 280th part of an Inch, being held in this Light, at the distance of about twelve Feet from the Hole, did cast a Shadow which at the distance of four Inches from the Hair was the sixtieth part of an Inch broad, that is, above four times broader than the Hair, and at the distance of two Feet from the Hair was about the eight and twentieth part of an Inch broad, that is, ten times broader than the Hair, and at the distance of ten Feet was the eighth part of an Inch broad, that is 35 times broader.
Nor is it material whether the Hair be encompassed with Air, or with any other pellucid Substance. For I wetted a polish'd Plate of Glass, and laid the Hair in the Water upon the Glass, and then laying another polish'd Plate of Glass upon it, so that the Water might fill up the space between the Glasses, I held them in the aforesaid beam of Light, so that the Light might pass through them perpendicularly, and the Shadow of the Hair was at the same distances as big as before. The Shadows of [Pg 319] Scratches made in polish'd Plates of Glass were also much broader than they ought to be, and the Veins in polish'd Plates of Glass did also cast the like broad Shadows. And therefore the great breadth of these Shadows proceeds from some other cause than the Refraction of the Air.
Widerspruch?
Eine Beugung von Licht müsste theoretisch bewirken, dass sich Licht vom Wandloch aus kommend hinter das Hindernis (Haar) ausbreitet und damit den Schatten verkleinert. Newton beschreibt aber genau das Gegenteil. Eine Interpretation wäre, dass Newton das Beugungsmuster mit zum Schatten zählte. Das Zitat "these Shadows have three parallel Fringes […] adjacent to them" klingt aber so, als unterscheide Newton zwischen Schatten und Randmustern. Theoretisch aber dürfte der Schatten mit Beugungseffekten eher kleiner als größer sein als es die Strahlenoptik fordert. Dieser Widerspruch bleibt hier ungeklärt.
Wie kann man das Experiment nachstellen?
Das Experiment wurde mit modernen Mittel in einer Lernwerkstatt in Aachen nachgestellt. Ein einfacher Laserpointer diente als Lichtquelle. An Stelle eines sehr dünnen Haares wurde die Klinge eines Teppichmessers (Cutter) verwendet. Damit ließen sich auf einer Wand sehr gut sichbare Intereferenzmuster erzeugen. Die Grundidee Newtons, ein schmales Hindernis in einem schmalen Strahl aus Licht, wurde dabei beibehalten. Das Experiment ist erklärt unter Doppelspaltexperiment nach Young ↗
Fußnoten
- [1] Isaac Newton: Opticks. Kapitel: The Third Book of Opticks Part I. Das Zitat stammt aus der Ausgabe des Jahres 1730 (London), ab Seite 317.