The experimental arrangement in Fig.1 shows schematically the side view of a prism and a white screen which can be positioned at A, B and C. A rectangular piece of black foil was glued to the left side of the prism in such a way that half of the surface of the prism was covered in this area.
If direct sunlight shone on the prism in a darkened room, images with colored border spectra appear on the screen: Fig. 2 at position C, Fig. 3 at position B and Fig. 4 at position A. In Fig. 5 the prism was removed and was replaced by a piece of cardboard with a cut-out of the same dimensions as the prism with the foil. This is how Fig. 5 appeared as a silhouette formed by the solar radiation and the shape of the cardboard. This shadow image appears on the screen in position A. The other three images appeared when the prism was used. They appear upwards from the shadow image, each at the height of the intersection of the dot-dash line with the screen in its respective position.
The steepness of the dash-dotted line resulted from the well-known laws of refraction in optics. With these laws, the displacement of the images can be calculated for a given geometric arrangement. The dash-dotted line is only a geometric auxiliary line and not a representation of a so-called light-ray.
The term light ray is used in optics published by Newton in the early 18th century. Newton assumed that light rays really exist and that they are preserved when passing through a prism. However, this idea of light rays, which were refracted through a prism is only a model, only an assumption, and not a proven fact. Therefore, the term light ray is not used in this presentation to describe optical phenomena.
The image in Fig. 2 appeared on screen C, when it was placed about 4 meters from the prism. In the left-hand part of Fig. 2 the lower and the upper border spectrum have moved so far together that a common spectrum with rainbow colors is created: blue, turquoise, green, yellow and red. Howeverr, this common spectrum only appears at a certain distance from the screen. The rainbow-spectrum is just a special case of border spectra.