Is shadow considered a color

Colored shadows

Colored shadows

Colored shadow with blue and white light. In the area of ​​the white shadow, a yellow color impression is created due to simultaneous contrasts.

Brief description
Basic structure of colored and colored shadows and selected applications in school lessons
Classification in the curriculum
Suitable for: Grade 9, 10, sec. II
Basic concept: Interactions
Implementation form Teacher demo experiment, student experiment
Number of experiments in this sub-category 1
Claim of the structure light
Surname: Mattias Stellbaum
Contact: @
University: Humboldt University of Berlin
Supervisor: Wiebke Musold
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Colored shadows illustrate many of the basic principles of color theory and color perception. As an interdisciplinary topic, colored shadows contain content from physics, biology, art, etc. The following experiment offers an opportunity to look at colored shadows in school lessons.

Didactic part

Colored shadows are a good application for interdisciplinary teaching.

"Interdisciplinary is a generic term for the different types of teaching that goes beyond the boundaries of your own subject." (see. [1])

With colored shadows, topics are taken up that are not only treated physically, but also strongly biologically and in a broader sense oriented beyond the natural sciences. As the name suggests, the topic of colored shadows includes the teaching of shadows. The pupils acquire a basic understanding of optical phenomena, especially geometric optics. This includes topics from physics and mathematics, namely geometry and proportionality (cf. [2]) contain. The calculations of lengths and areas, as well as, in a broader sense, angle functions are used here.

Another focus for the colored shadows is the theory of colors, above all color mixing and color perception. The color mixture contains references to the properties of visible light, which is part of the physics department, as well as to painting and the meaning of colors, which is of particular interest in art (cf. [3]). Additive and subtractive color mixing as well as complementary colors are important here.

Color perception is of interest in sensory physiology, i.e. in the field of biology. This is where the eye stands (cf. [4]), sensory perception (cf. [5]), and neurobiology (cf. [6]) in the centre. The structure and function of the eyes play a fundamental role in understanding. Neurobiology is very important because in this experiment colored shadows are placed in connection with simultaneous contrasts. This means that colored shadows can be used both as a teaching topic for secondary level I and as a teaching topic for upper secondary school.

In a broader sense, the colored shadows also create connections to the German and History departments. In his work, Goethe describes the theory of colors (cf. [7] and [8]) in detail the phenomenon of colored shadows that he observed on his trip to the Harz Mountains.

Experiment instructions


Fig. 1: Schematic drawing of the experimental setup with two lights and an opaque object

Device list:

- 2-3 AC / DC power supply unit 0 ... 15V 52150 Leybold

- 2-3 light box, halogen, 12V / 20W

- different color filters (red, green, blue)

- 1 opaque object as a shadow source

The experimental setup for creating colored shadows is shown in Figure 1. The experiment is carried out with 2 lights, optionally also with three lights. In addition, an opaque object is required that is intended to provide a shadow.

The lights are arranged next to each other so that the projection surfaces overlap as completely as possible. The lights are supported with feet. The lights are connected to the AC / DC power supply units by means of cable connections. The power supply units are set to 14V. Depending on which color filter is used, the applied voltage must be varied (by a few V). This is due to the fact that filters that allow short-wave light to pass through are less light-intensive than filters that allow long-wave light to pass through. The voltage would therefore have to be increased by approx. 2V for the blue filter and reduced by approx. 2V for the red filter so that the intensities of the lights do not differ too much.

The object casting the shadow is placed in front of the lights so that two penumbra are formed on the projection surface. The object can either be fixed or held by hand. A white canvas (if available) or a white wall can serve as a screen. "The question of the relationship in which the" figures "of shadow giver and lamp" mix "in the shadow image [11, p. 94], so that sometimes the figure of the shadow giver, sometimes the figure of the luminaire in the shadow image, is more prominent to a consideration of the spatial conditions and in particular the relative sizes of the shadow and luminaire geometry. " (see. [9]). Grebe-Ellis states that the shadow image can be shown better, the smaller the shadow source is in relation to the size of the luminaire. Experimental personal experience shows that with an object the size of a saucer, the phenomenon of colored shadows with core and penumbra can be demonstrated very well if the shadow generator is a little more than 1/3 of the total length between the lamp and the shade from the lamp. The total distance should not be less than 1m.


Fig. 2: Colored penumbra to create a color wheel

First, the experiment is carried out with two lights. After the power supply units have been switched on, 14V is set. The room should be darkened as much as possible. A filter is now placed in front of a lamp. The colors of the white lamp and the colored lamp are mixed additively on the projection surface. Now the object is brought between the lights and the projection surface. Two penumbra are created on the projection surface. One shadow is shown in the color of the filter, the other shadow apparently in the complementary color of the surroundings. The filters are then changed one after the other, which leads to different color impressions on the white surface. If the experiment was carried out with two lights, a third light can be added. The principle is then the same as with two lights, but different color impressions are created because the colors are now mixed by two color filters used.

A distinction must be made between colored and colored shadows. The colored shadow is the shadow of the white light. It is illuminated by the colored lamp and thus colored. The colored shadow is irradiated by the white lamp and, in connection with the surrounding surface color, there is a contrast enhancement, which makes it appear colored.

Westphal (see [10]) explains three concepts to explain the colored shadows:

1. "Colored shadows as an example of simultaneous color contrasts",

2. "Colored shadows as a result of color constancy" and

3. "Colored shadows as colored darkness".

In this experiment, reference is preferably made to the first concept “Colored shadows as an example of simultaneous color contrasts”, as this concept relates to teaching content such as contrasts and optical illusions and therefore enables good transitions to other subjects in the curriculum. Colored shadows are described as the physiological reaction of the eyes to simultaneous contrasts.

Fig. 3: Colored shadow with blue and white light
Fig. 4: Colored shadow with red and white light
Fig. 5: Colored shadow with green and white light


In this experiment, the color impressions of the colored shadows under the influence of the ambient color in two luminaires were examined. Using a color wheel, as shown in Fig. 2, you can see which color impressions arise under which conditions.

A white area surrounded by a colored area appears in the complementary color of the surrounding area color. The complementary color is understood to be the color that does not contain any part of the surrounding color. In the case of blue, the complementary color would be yellow, in the case of red it would be cyan and in the case of green it would be magenta. The rules of color mixing are followed. The complementary colors can be examined in the color wheel. The color opposite in the color wheel, seen from a basic color, is always its complementary color, i.e. the color that has no part in the basic color.

The color impressions are shown in Fig. 3 to 5.


As already described, an apparently colored impression is created on the white surface. The color filter in front of the other lamp influences the color perception of the eyes.

There is a neural contrast enhancement (see [11]). The contrast enhancement involves different input excitations from neighboring sensory channels. A sensory channel forwards the information received by several receptor cells to the brain.

As an example, consider a white area surrounded by a blue area. One sensory channel receives a stronger blue signal than the next one. Due to lateral inhibition, the first sensory channel will inhibit the second more, so that the output messages of the second sensory channel are disproportionately weakened. The input contrast is therefore passed on more intensively. For our "blue example" this means that the blue receptors are stimulated to a greater extent when looking at the blue surface, but are disproportionately de-energized when looking at the white surface and we are comparatively more sensitive to the green and red components of the White area react. The area therefore appears yellow. Mach’s pictures illustrate this principle very well. If the white surface is viewed separately (e.g. through a tube), it appears white.

Cameras carry out a "technical" contrast enhancement with the aim of imitating human color perception. The same color impressions appear in a photograph as in reality. The difference to the eye is that the colored surface of the colored shadow on the photograph, viewed separately, remains colored and does not turn white, as in reality, because the colored impression was recorded as such.

safety instructions

Since the room is darkened as much as possible, it should be ensured that the structure is space-saving in order to limit the risk of injury and tripping.


  1. ↑ Labudde, Peter (2010): Fachdidaktik Naturwissenschaften - 1st - 9th school year, 1st edition, main UTB
  2. ↑ Framework curriculum for lower secondary level physics, 1st edition (2006), Senate Department for Education, Youth and Sport, Berlin, p. 28
  3. ↑ Framework curriculum for lower secondary level physics, 1st edition (2006), Senate Department for Education, Youth and Sport, Berlin, p. 52
  4. ↑ Framework curriculum for secondary level I biology, 1st edition (2006), Senate Department for Education, Youth and Sport, Berlin, p. 38
  5. ↑ Framework curriculum for secondary level I biology, 1st edition (2006), Senate Department for Education, Youth and Sport, Berlin, pp. 38, 61
  6. ↑ Framework curriculum for the upper secondary school biology, 1st edition (2006), Senate Department for Education, Youth and Sport, Berlin
  7. ↑ Goethe, Johann-Wolfgang (1810): Color theory, Volume 1, Verlag Freies Geistesleben 1979
  8. ↑ Goethe, Johann-Wolfgang (1810): Color theory, Volume 2 - From the colored shadows (1792), Verlag Freies Geistesleben 1979
  9. ↑ Grebe-Ellis, Johannes (2010): Shadow images like reading characters - an image-optical study according to Kepler
  10. ↑ Westphal, Nico (2008): The connection between the phenomena of colored shadows and the color appearances in Edwin H. Land's projection experiments
  11. ^ Reichert, Heinrich (2000): Neurobiology, 2nd edition, Georg Thieme Verlag

See also

Color mixing