Dietrich Zawischa

Deutsche Version

Simple observations with a prism

Little is needed for the following experiments: In addition to a prism only a piece of black paper and white and coloured paper strips.
The following photographs objectify subjective observations. On the left hand side the object is shown, to the right its appearance if viewed at through a glass prism.
It is, however, not possible to reproduce spectral colours faithfully. In the visual impression, the blue end of the spectrum is rather violet-blue, and the green is a deeper green. The pictures shown here cannot substitute the real experience.

Put a thin strip of white paper on a dark surface and look at it through a prism. You have to look in a different direction, and due to the differences in the refractive index for the different wavelengths of light, the strip now looks coloured. If the strip is sufficiently narrow, there are essentially only three colours to be seen: red, green and blue-violet. This is a special case of the phenomenon known since the 19th century under the name of Bezold-Brücke shift.

Thus we perceive the spectrum of white light, if it is rather dim, as made up of only three bands, red, green, and violet-blue, with hardly perceivable transitions.

For the photographs, the arrangement was as shown in the adjacent sketches, only the eye was replaced by the camera.

Three rays of light from the white stripe are shown. The prism splits them up because the higher the frequency, the more the light waves are refracted. Only three refracted rays are drawn each time, but in reality each ray creates a fan of rays. (The drawing is strongly exaggerated.)

It is easier and clearer to draw only rays that reach the eye, if you want to visualize the geometry (lower picture). The "white" rays are indeed superpositions of rays of all the wavelengths present in the light, and if only the portion ultimately reaching the eye is drawn, it should not be concluded from this that the rest is not there.

The next pair of images shows the comparison of yellow and white narrow paper strips.

In the lower red band, no difference is seen, and also the yellowish green is scarcely weaker on the right hand side. But the blue band is almost completely absent. This part of the spectrum is absorbed by the yellow paper.

This enables us already to sketch the remission curve of the yellow paper. One has only to know that the "blue border" between violet-blue and green is at approximately 480–490 nm and the "yellow border" between green and red at about 570 nm:

For the pictures below, narrow stripes have been partially coloured with the printing primaries cyan, magenta, and yellow, then cut out and placed on black paper.

Can you guess how narrow stripes of other colours look through the prism?

Let us now promenade part of our way following the footmarks of J.W. von Goethe. Looking through a prism at a white wall, Goethe was highly disappointed. But then he found colours along the border between light and dark areas.

Looking at a broader white strip on dark background, the central portion remains white, while the borders become coloured fringes: yellow – red the one, ice-blue (cyan) – purple-blue the other one.

The central white is easily understood: light reaching one point at the eye's retina does not originate from a single point of the surface looked at, but instead from different points along a line. This is sketched to the right for three selected rays, which have been coloured according to their wavelength for simplicity.

The explanation of the coloured fringes is simple too. (In the above sketch one may keep the light paths unchanged, but think the white area moved to the left or to the right – if no ray emerges from the dark, which colour is seen if the viewing direction is not changed?

There is nothing new in the next pair of images.

But if the broad central areas are replaced by narrow strips, there is something new, at least on the right hand side of the image.

In the left part of the image, the white strip is decomposed into blue, green, and red bands (with a narrow yellow transitional region between green and red), while in the right half, the black strip decays into yellow, purple (magenta), and cyan bands (with a narrow blue region between purple and cyan).

These and similar observations misled Goethe to attribute the leading part of colour generation to the boundary between black and white – or between light and darkness – (while denying the possibility that white light could be split into different colours) and to assume symmetry between light and darkness.

However, there are light sources, but no darkness sources. How the latter would work is revealed in a poem by Christian Morgenstern:

Die Tagnachtlampe Korf erfindet eine Tagnachtlampe, die, sobald sie angedreht, selbst den hellsten Tag in Nacht verwandelt.		The Day–Night Lamp Korf invents a day-night lamp which, when turned on changes even the brightest day into night.
Als er sie vor des Kongresses Rampe demonstriert, vermag niemand, der sein Fach versteht, zu verkennen, daß es sich hier handelt –		When presenting it at a congress on the stage, nobody who has brains, can misconceive that it is –
(Finster wird's am hellerlichten Tag, und ein Beifallssturm das Haus durchweht) (Und man ruft dem Diener Mampe: »Licht anzünden!«) – daß es sich hier handelt		(It gets dark on the bright day, and thundering applause blows through the house) (And one calls to janitor Mampe: »enkindle light!«) – that it is
um das Faktum: daß gedachte Lampe, in der Tat, wenn angedreht, selbst den hellsten Tag in Nacht verwandelt.		clearly the case that the lamp indeed, when turned on, changes even the brightest day into night.

Christian Morgenstern knew Rudolf Steiner well, who admired Goethe and particularly his "Farbenlehre" (colour theory). I guess that it was this acquaintanceship which stimulated Mr Korf to invent the day-night lamp.

Objective experiments

Objective experiments (which may be observed by several people simultaneously) require somewhat larger effort. In the times of Newton and also of Goethe, the only appropriate source of light was the sun; today other ones are available. For the next image, a slide projector has been used, the slide being replaced by a narrow slit. The light is refracted by a prism to a screen on the wall.

It should be pointed out once again, however, that it is impossible to reproduce a spectrum in photography or image quite faithfully. So the picture only shows approximately what can be seen. But note that not only red, green and violet blue are to be seen here at sufficient brightness, but also orange, yellow, cyan and "indigo" (blue) with continuous transitions.

In the image below the beam of light is passing a second narrow slit, then the prism resting on white paper, and finally falls on a screen (a piece of white cardboard).

dispersion of light by a prism

Back to the text "Introduction to colour science" or to the page: “Seeing, measuring, and rendering colours”.