Edge spectrum

from Wikipedia, the free encyclopedia
Edge spectrum of a dark stripe on a light background (left) or a light stripe on a dark background (right)

An edge spectrum is a colored border that is created on a high-contrast edge when viewed through a prism . In the case of black-and-white contrast, either a red-yellow or a purple-blue color fringe is seen. In the former, the refracting edge of the prism is on the white side of the black-white transition, the result is also called the "warm edge spectrum", in the other color fringe the refracting edge of the prism is on the black side, which is also called a "Cold edge spectrum".

The edge spectrum is also referred to as the result when two such oppositely colored hems move together and partially overlap, so that another color is created between the previous colors. When looking at a narrow dark stripe in front of a light background, the so-called Goethe spectrum emerges . This contains what Goethe called “peach blossom” magenta (or purple ), a color that does not appear among the spectral colors . When looking at a narrow light strip, green is the fifth color ( Newton spectrum ).

Edge spectra were described as early as the 17th century.

history

Kenelm Digby described in 1644 in the chapter "Of luminous or apparent colors" ("Of light and body colors") of his work Two Treatises that he discovered the phenomenon of colored fringes in a prism experiment. This experiment would have been made by Thomas Harriot about half a century earlier . To explain the red and yellow fringing in particular, Digby referred to Aristotle and wrote that the images of the white (light) and black (darkness) side are differently deflected and sometimes overlap, the creation of the colors is, as it were, due to the mixing of Light and darkness reduced. The art historian Karin Leonhard evaluates Digby's attempts to explain it in such a way that "... these color phenomena, even if they were already understood as the result of prismatic scattering, remained in an intermediate area between the Aristotelian theory of color mixing and contemporary corpuscular light conceptions" (for example in the color system of Franciscus Aguilonius ).

The fact that white light is the sum of all spectral colors, which can be made individually visible with the help of a prism, was recognized and described by Isaac Newton around 1700 . Instead of looking directly through a prism, it generated the light spectrum of all the colors contained in white sunlight on a screen. He was able to make the two color fringes visible separately by opening the gap in front of the prism almost to its width.

Almost a century later, Johann Wolfgang von Goethe looked through a prism as part of his work " On the theory of colors ". When he looked through a window into the light-gray sky, he saw different colored lines on both sides of the window bars. When viewed from a greater distance, the darkness between the seams had disappeared and a closed color spectrum could be seen. At that moment it became clear to him, according to Goethe, that Newton was wrong. In Goethe's view, colors are created “not through the division of white light”, but only “through the interaction of light and darkness”. The edge colors are the result of a movement of white and black surfaces on top of each other.

The phenomenon

Figure 1:
View and photography through a prism
Figure 2: Edge spectra
left: on a dark stripe (bar → Goethe spectrum ),
right: on a light stripe (slit → Newton spectrum )

The subjectively perceptible phenomenon can be documented photographically. The function of the eye (lens and retina ) is taken over by the camera (lens and photo film or digital image sensor ) (Figure 1). Figure 2 shows photos of black paper strips glued to white paper ( bar , left column) and white paper strips glued to black paper (right column). They are recorded with and without a prism and placed one below the other: first and second row. Rows two to four contain images of successively narrow strips. The same change in impression is also achieved with a successive increase in the distance between the object and the observer.

In contrast to the light spectrum, the colored edges only consist of two to three colors. These are the long-wave colors (red and others) if there is white below the edge (position of the prism as in Figure 1). If white is above the edge, the border consists of short-wave colors (purple and others). Both hems of a strip move together when it becomes narrower. Eventually, both hems flow together into a single color image.

The color image created by the gap is similar to the light spectrum and is sometimes called the Newton spectrum despite the lower color range (Figure 2, bottom right). Accordingly, the color image created by the bar is sometimes called the Goethe spectrum (also reverse spectrum , Figure 2, bottom left).

The explanation

The edge spectra seen by Goethe do not refute the “division of white light”, but can be explained as a result of its spectral decomposition.

Edge spectrum of a gap or white bar

The slit used in a spectroscope must be narrow so that the slit images generated with one spectral color each do not significantly overlap, so that the color spectrum remains resolved. Therefore, the tests can only be carried out with a strong contrast between the light source and the ambient light or in the “dark chamber”.

The gap images (bar images) of many different colors are so wide in the edge spectrum because of the relatively wide gap (white bar) that they largely overlap and when added together, a predominantly white image results. Only the colors red and violet in the two edges of the spectrum are retained in the two edges. Their neighboring colors orange and yellow or blue and cyan become increasingly lighter and lose themselves in the white center.

If the gap becomes narrower, the red-orange and the blue gap image disappear from the center. One approaches the usual spectrum seen with a spectroscope, in which green appears in the middle instead of white. The previously existing total mixture (white light) is reversed, a separation takes place.

Edge spectrum of a black bar

As the beams become thinner, one edge of the two infinitely wide gaps that enclose it overlap (so-called half - gaps seen from the edge of the beam). First a red and a purple sub-image fall onto the previously black center. Magenta becomes visible, which can easily be explained as the result of an additive color mixture of the two superimposed pure border-edge colors red and violet. In contrast to the green of the white bar, the magenta becomes increasingly lighter as the bar becomes narrower. All other colored partial images are added, and finally only a white surface is perceived. The initially recognizable decomposition of white light into colors by the prism is canceled. The colors have been totally mixed again.

literature

  • Johann Wolfgang von Goethe: On the theory of colors. Tübingen 1810 ( online ).
  • Maurice Martin: The controversy over the theory of colors. Novalis Verlag, 1979, ISBN 3-721-40055-0 .
  • Ingo Nussbaumer : On the theory of colors - discovery of the messy spectra. edition splitter wien, 2008, ISBN 978-3-901190-38-4 .

Individual evidence

  1. ↑ a b From: Olaf L. Müller: More light: Goethe and Newton in a dispute about colors , extracts online at www.farbenstreit.de
  2. ↑ At the time of Goethe, the later purple (today mainly magenta) was called "peach blossom" .
  3. ↑ Sabine Schimma: aesthetics and experiment in Goethe's color studies. Böhlau Verlag, 2013, p. 399, doi : 10.7788 / boehlau.9783412216184
  4. ^ A b Digby, Kenelm: Two Treatises, in One of Which the Nature of Bodies / in the Other, the Nature of Man's Soul is Looked Into / in Way of Discovery of the Immortality of Reasonable Souls. Paris, 1644 and in various reprints, e.g. B. Online at: biodiversitylibrary.org.
  5. ↑ a b Karin Leonhard: Image fields. Still lifes and natural pieces from the 17th century. Oldenbourg Akademieverlag, 2013. ISBN 978-3050063256 . P. 384 f.
  6. ↑ Isaac Newton: Opticks or a treatise of the reflections, refractions, inflections and colors of light . London 1704
  7. ↑ The light spectrum resulting from a narrow gap is richer in color than the spectrum resulting from viewing a narrow white strip. It is also richer in color than the Goethe spectrum that emerges when looking at a narrow black stripe.
  8. ^ Isaac Newton: Optics. Thun and Frankfurt / M., 1996 and 1998, pp. 104-06, Prop. VIII. Aufg. 3. Cf. Ingo Nussbaumer: On the theory of colors - discovery of the messy spectra . edition splitter wien, 2008, ISBN 978-3-901190-38-4 , p. 62
  9. ↑ a b Ingo Nussbaumer: About the peculiarity of complementary spectra , lecture manuscript at “Working days for physicists and physics teachers” of the Anthroposophical Society / Free University of Spiritual Science, Natural Science Section Dornach, from p. 4 (PDF; 768 kB).
  10. ↑ The resulting colors are complementary to those of the Newton spectrum .
  11. ↑ Hermann von Helmholtz : About Goethe's scientific work.
  12. ↑ a b c Lutz Wenke, Friedrich Zöllner, Manfred Tettweiler, Hans-Joachim Teske: Sun and truth freely after Goethe. Carl Zeiss Vision GmbH (PDF). ( Memento of the original from February 25, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.vision.zeiss.de
  13. ↑ a polemical term used by Goethe for Newton's working method