Home » Sylvie Zanier and Julien Delahaye : teaching color at school to introduce children to the scientific approach

Sylvie Zanier and Julien Delahaye : teaching color at school to introduce children to the scientific approach

Do you know the “1, 2, 3, Couleurs ! ” project? Probably not as it is a French project devoted to colors.

The search for color filters documented by their transmission spectrum led me to its site. Thanks Google! Behind this project, there is a micro-company that sells educational material to perform experiments on color and scientific pages to “marvel, experiment and understand”. What a coincidence and complementarity with the mission of my blog dedicated to color!

Given the quality of the scientific and educational content, I told them about this series of articles on teaching color. Once the contact was established, I was delighted to discover the richness, the care of writing and the enthusiasm of the two writers, Sylvie Zanier and Julien Delahaye.

Sylvie is a teacher at the University of Grenoble Alpes while Julien is a researcher at the CNRS.

The adventure of “1, 2, 3, Couleurs !”

Both physicists, Julien and Sylvie fell into the pot of color in 2005; it was the World Year of Physics. For them, as for many scientists, color was then primarily associated with electromagnetic waves, those that our eyes are able to perceive. But the questions relating to the color as matter or as sensation (how and why do we see colors?) had not yet titillated them.

From interventions to demonstrations, from scientific events to science festivals, their passion for color develops. The subject turns out to be favorable to the transmission of the scientific approach. The proposal of training on the color is then refined and takes shape in an educational course for the House for the Science in Alpes Dauphiné; we are in 2015, the international year of the light. On this occasion, a contest for elementary school will plunge them into the questioning of teaching color to children, thus renewing their inspiration.

In January 2017, Céline Cardeilhac joined them to create the micro-company “123 Couleurs”, aimed at understanding color and its mysteries through the practice of simple scientific experiments.

But now, I leave you in the hands of Sylvie Zanier and Julien Delahaye who wrote the rest of this article.

Please note that many links provided by the authors lead to the scientific pages of “1,2,3 Couleurs ! “, and are therefore in French.

A scientific training on color for school teachers

As scientists, we have been participating for more than ten years in numerous mediation actions on the theme of color. We intervene with various audiences, in particular schools, from kindergarten to university.

Thus, between 2015 and 2018, we have been asked by the House for Science in Alpes Dauphiné to offer a training dedicated to the scientific aspects of color to school teachers of the Grenoble academy.

The Houses for Science were born in 2012, at the initiative of the French Academy of Sciences. They are coordinated by the foundation La Main à la Pâte and aim to help primary school teachers to improve their science teaching practices. In particular, they propose a continuing education offer involving actors from the world of research (public and private).

In this article, we describe the typical course of our training, which in its longest form extends over two days. It is aimed at primary school teachers already in service, with a wide range of profiles, most of whom have not studied science. The objective is therefore to provide knowledge on the subject, ideas for activities to be carried out in class with the pupils, but also for some, to (re)awaken the desire to do science in class.

After a day of training, the table is covered with a pretty colorful mess. The pleasure of experimenting is not only for children (© Photo : Patrick Arnaud) !

Why is color a great topic for doing science at school?

First of all, color is a subject that allows a real scientific practice from a very young age. Numerous activities involving more or less complex protocols and reasoning are possible, without the need for sophisticated or expensive equipment.

Secondly, it is an everyday subject that affects us all. We see “in colors” and in our modern world, colors are omnipresent: clothes, posters, luminous screens, etc. But paradoxically, we usually don’t really know what seeing in color means, and we don’t know how colors can be technically (re)produced. The motivations to know more are therefore not lacking: for the youngest children, one can approach the subject via the mixtures of colored materials (paints, felt-tip pens, etc.), for the slightly older children, via the reproduction of colors on luminous screens.

Finally, it is a subject that allows to build bridges between different disciplines, scientific and non-scientific, that usually do not talk much to each other. An artist will find an interest for his practice in adopting the vision of a scientist, and conversely a scientist will have much to learn from the vision of an artist. Every teacher can therefore find a personal motivation to do science on the theme of color at school.

The objectives of the training

From the sole point of view of color, the objectives of this training are twofold. First of all, it is to make the participants aware that color mixtures follow two distinct types of rules, depending on whether one mixes colored materials (subtractive synthesis) or colored lights (additive synthesis). It is then a question of giving them keys allowing to understand where these rules come from, and which are the links between them.

More generally, this training is also an opportunity to show the power of the scientific approach to provide answers to simple questions: why does a mixture of yellow and cyan give green in paint? Why is it different if you mix colored lights? How are colors reproduced on printed media? On luminous screens? A lot of scientific knowledge is also introduced: light spectra, wavelength, absorption and emission spectra, structure of the eye, interactions between light and matter and its famous “ion” words (diffusion, reflection, refraction, diffraction, dispersion).

Why do yellow and cyan paints turn green when mixed? This is one of the questions to which the scientific approach allows to bring elements of answer.

An approach based on experience

The experiment is at the center of our approach and the progression in the training is done by a succession of questions / experiments / discussions / explanations. Whenever possible, participants are invited to perform the experiments themselves, and when explanations are given, an experimental “proof” is proposed. Without this practical step, the risk is to miss the subtleties inherent to a “real” experiment, not to mention the possible technical difficulties of implementation.

It is also an opportunity to show how, from the complexity of reality, one can derive rules and models, which allow for progress in the understanding and prediction of phenomena.

Finally, it is often more interesting to remember the method that allows us to find a result than the result itself. This is one of the specificities of the scientific method: everyone has (in principle) the possibility of verifying, and even possibly questioning, the explanations that are proposed.

For the practical application in class and the deepening of certain notions, the participants can rely on the website 123couleurs.fr that we have been developing since 2015, and which aims at proposing experiments, explanations and material resources to as many people as possible. Most of the experiments proposed during the training are included on the site.

The home page of the 123couleurs.fr website where are gathered resources (experiment protocols, explanations, material) to carry out scientific activities on the theme of color.

Day 1: Introduction and establishment of color mixing rules

What words do you associate with the word “color”?

At the beginning of the training, each participant is asked to write down on a piece of paper three words they associate with the word “color”. These words allow us to know what the participants have in mind; these words reflect their own culture: paint and pigments for those who have a more artistic culture, wavelength and spectrum for those who have a more scientific culture.

Examples of words associated with the word “color”. These words are classified in three different categories, matter, light and vision, with some overlaps

How are the colored materials mixed?

To answer this question, participants are divided into small groups, each group exploring the mixing of colored materials with a different medium: colored plastic filters, food coloring, felt pens, modeling clay.

The pooling of the different results makes it possible to realize that, apart from a few nuances, the rules of mixing are comparable from one medium to another. These rules are those of the so-called subtractive synthesis of colors. They are also an opportunity to define what is called a primary color.

From a methodological point of view, the exploration of mixtures of colored materials is particularly interesting with children: it requires rigorous organization, both in the choice of mixtures that are made and in the way to note the results obtained.

The colored filters are great to establish the rules of mixing colored materials. We find on this image, where pieces of filters are partially superimposed, the primary colors of the subtractive synthesis of colors (yellow, cyan and magenta), the secondary colors (red, green and blue), which can be obtained by mixing two primary colors (red = yellow + magenta, green = yellow + cyan, blue = cyan + magenta), and the couples of complementary colors (a primary and a secondary which give black by mixing) as for example cyan and red.

How are these rules used in practice?

These rules are for example used to reproduce the colors on printed supports. The technique of quadrichromy, which is the most widespread, uses primary inks yellow, cyan and magenta, to which is added black ink which makes it possible in particular to increase the contrasts.

To reproduce the colors, this technique calls upon superimposition of spots of inks for the most saturated colors, but also with juxtapositions, which one cannot discern with the naked eye (one speaks in this case of optical mixture).

A digital microscope makes it possible, for a few tens of euros, to discover how the colors are reproduced on the printed supports. On a green of intermediate saturation, printed with a cheap domestic printer, one observes spots of cyan and yellow inks distributed randomly on the surface of the sheet, partly juxtaposed and superimposed (some spots of magenta are even visible). The image is about one millimeter wide.

The rules of subtractive synthesis are also used to obtain a wide range of colors from a reduced number of dyes.

The chromatography technique makes it possible to carry out a “magic” operation: to separate dyes that have been mixed. The different participants are invited to read at home the protocol which is described in detail on the 123couleurs.fr website, then to test it on all the felts in a box. The results obtained and the difficulties encountered are discussed collectively at the beginning of the second day of training. Chromatography is an opportunity to introduce children to a real laboratory technique, which requires care and method.

Chromatography allows us to identify, with simple coffee filters or blotting paper, which dyes are in the inks of a box of felt pens. In this example, five different dyes are visible in the inks of our twelve markers, ten minutes after dipping the lower end of a sheet of blotting paper in salt water. In particular, we find that the inks of the green markers are made from mixtures of yellow and cyan dyes, while the inks of the blue markers contain cyan and magenta dyes.

How do the colored lights mix?

If the rules for mixing colored materials are familiar to many of us, especially to young children who are used to handling paint, the same cannot be said for mixing colored lights. But when we test these mixtures with three sources of “white” light and a set of colored filters, we realize that the rules established previously are no longer valid: the colors yellow, cyan and magenta can be obtained by mixing (respectively of green and red light, green and blue, and blue and red) and are therefore no longer primary colors.

The new primary colors (defined by the fact that they can not be obtained by mixing) are red, green and blue, which give white when we mix all three together. These new rules are those of the so-called additive synthesis of colors, whose symmetry with the rules of subtractive synthesis does not seem to be random: the primaries of one are the secondaries of the other; in one case, we start from black and we arrive at white (with the mixtures of lights), in the other it is the opposite (with the mixtures of paints).

In this room of the “Palais de la Découverte” in Paris, three red, green and blue lamps hang from the ceiling and light the floor. We can clearly see the new colors obtained by mixing, where the different discs of colored lights overlap.
By placing your hand in three beams of red, green and blue lights, in front of a screen or a white wall, you can observe beautiful colored shadows. A playful way to study with children the rules of mixing colored lights and to think about the notions of shadows and penumbra.

How are these rules used in practice?

We use the rules of mixing colored lights without even knowing it every time we look at a bright screen (computer, television, smartphone, etc.). And as for printed media, a digital microscope allows us to easily visualize how it works: packages of three light rectangles, red, green and blue, called “pixels”, cover the entire screen.

By adjusting the light intensity of the red, green and blue rectangles of each pixel, we can reproduce (almost) all the colors we know. This is the RGB coding of colors. At the distance at which we usually look at the screens, these rectangles of light can not be discerned and the mixture occurs in our eye (here again it is an optical mixture).

In this image, a digital microscope has been placed on a region covering a thin vertical yellow line and a white background. The white background is obtained by turning on the red, green and blue rectangles at their maximum intensity, and the yellow line (which is three pixels wide) is obtained by turning off the blue rectangles.

How do the colors mix on a spinning top?

The first day ends with this open question, with the implication that colors on a spinning top mix like colored materials or like colored lights. As with the chromatography of felt-tip inks, each participant is invited to test this other way of mixing colors at home and the results obtained by each are discussed with the whole group at the beginning of the second day.

One is tempted to answer this question “like colored matter” because the colors of the disks are obtained with dyes or pigments (here, the disks were colored with felt pens). But as this photo shows, the answer is different: when the red, green and blue sectors start to turn and “mix” two by two, we obtain yellow, cyan and magenta rings, according to the rules of additive color synthesis.

Day 2: Explanation time

The second day of training will provide answers to the many questions that arise from the first day’s observations. Why three primary colors and not four? Why these and not others? Why this perfect symmetry between the rules of additive and subtractive synthesis? And why are they called “additive” and “subtractive”? (what is added or subtracted?)

How can we tell that an apple is red?

We propose to return to simple things and to ask ourselves what is necessary to see colors. We then realize that we need white light (a colored light modifies the color of the objects); an object, i.e. matter, which reflects a part of the light towards us; eyes and a brain. And it is by analyzing these various elements (what is white light made of? what does matter do to light? how does our visual system work?) that we can hope to answer all the previous questions.

We can say that an apple is red because it is illuminated by white light, that it reflects some of this light back to our eyes and because this light is finally analyzed by our visual system.

First step: what is white light made of?

We saw on the first day that it is possible to obtain white light by “mixing” red, green and blue light. But what about the white light that we use for lighting, whether it is natural (direct or indirect sunlight) or artificial (lamps)?

To obtain this valuable information, it is necessary to do what is called a light decomposition experiment. And if the decomposition by a prism is the most famous technique (Newton used it in 1666), it is generally not the easiest to implement.

We therefore present to the participants different experiments and objects that allow to easily decompose white light in class: water basin and mirror (water prism), compact disc, diffraction glasses. These experiments make it possible to see that the white light of the sun or of a halogen lamp is composed of a continuous mixture of “pure” colored lights going from purple to red, while passing by blue, cyan, green and yellow, a gradient commonly called “the colors of the rainbow”.

This is an opportunity to talk about the physical nature of light (light is an electromagnetic wave) and to introduce the concepts of spectrum and spectral colors. It is also an opportunity to recall that not all the colors we know are in the rainbow: in particular, magenta (which is called “extra-spectral color” for this reason) is not found there.

The diffraction glasses are a very playful and cheap way to decompose white light. By looking with these glasses at a small light source in the dark, we see spectra appearing on both sides of the source, i.e. gradations of the colors of the rainbow. Even if the physical mechanisms of interference and diffraction at the origin of this decomposition are difficult to understand for children, the show and the emotion are there!

Second step: what does matter do to white light?

When an object is illuminated by white light, it receives all the pure “light” colors of the rainbow. And it is the capacity that this object will have to return or to absorb certain spectral components of the received light, which will give him its color (if we limit ourselves to the colors known as chemical). Using one of the decomposition experiments described above and placing colored filters in front of the white light source, it is possible to see which parts of the spectrum are absorbed by the filters and which parts are transmitted. This property can then be generalized to all colored objects, including those that are not transparent. It is thus understood that a yellow object preferentially absorbs the blue part of the spectrum (violet-blue), a cyan object the red part of the spectrum, and a magenta object the green part of the spectrum. A primary color of the mixtures of colored materials thus absorbs a primary color of the mixtures of colored lights.

By using diffraction glasses and placing colored filters in front of a white light source, one can very easily visualize which parts of the spectrum are transmitted and absorbed by the filters. On the left, white light from a flashlight is seen through a 2D diffraction grating. On the right, a magenta filter is placed in front of the lamp: the green part in the center of the spectrum “disappears” and there remains a mixture of red and blue, which we see magenta.
With a spectrometer, we can measure precisely what percentage of the incident light intensity is transmitted by colored filters depending on the wavelength. A cyan filter absorbs the “red” part of the spectrum (the transmission coefficient is very low for wavelengths above 580 nm), a magenta filter, the “green” part of the spectrum (wavelengths between 490 and 580 nm) and a yellow filter, the “blue” part of the spectrum (wavelengths below 490 nm). By superimposing the yellow and cyan filters, the blue and red parts of the spectrum are absorbed, the green part is (partially) transmitted: we see green. If we add a magenta filter, which absorbs the green part of the spectrum, there is (almost) no light transmitted: we see black.

We speak of “subtractive” synthesis for mixtures of colored materials because we refer to the fact that colored materials “subtract” pure “light” colors to the white light they receive. For example, by mixing a yellow material with a cyan material, the blue and red parts of the spectrum are absorbed, and only the green part is reflected or transmitted: we see green. From a methodological point of view, these experiments highlight the power of logical and deductive reasoning, reasoning that can be done even with very young children because it does not require any equations (everything can be seen with the eyes!).

At the end of this sequence, the teachers are invited (by provocation) to review their way of talking about colors at school: instead of telling the children “take a white sheet of paper”, they should say: “take a sheet of paper that reflects all the colors of the rainbow”. Instead of saying “color this sheet yellow” they should say “take the blue out of the light”, instead of saying “color this sheet cyan” they should say “take the red out”. The children would then understand why by coloring it with both yellow and cyan paints, the leaf becomes green. We invite them to reason from the point of view of light, not matter.

To paint in yellow is to absorb the blue region of the light spectrum (© Drawing: Clara Mallet-Burgues)

Third step: how do we see? And above all, how do we see colors?

At this point, we have understood that the color we perceive is directly related to the spectral composition of the light that enters our eyes. We understood that a primary “matter” color absorbs a primary “light” color. But we still haven’t understood why there are three primary colors (and not four or five) and why these and not others.

It is the moment to remember that color is first and foremost a sensation (some would contest this formulation by putting forward the cultural dimension of color) which is done through our visual system (eyes, brain). It is also the moment to introduce a simplified model of our visual system, which makes it possible to understand a great part of the phenomena described previously: we perceive the colors via millions of different sensors called cones and placed at the bottom of the eye, on the retina. These cones are of three types and three types only, with different zones of sensitivity in the spectrum of the light: a first type (B) is sensitive in the range of the blues, a second (G) in the range of the greens, and only the third type (R) is clearly sensitive in the range of the reds.

The spectral responses of the three types of cones called “Blue” (B), “Green” (V) and “Red” (R) present on the retina show us the limits of the simplified model described in the text. We note in particular that the shift in wavelength between the responses of the V and R cones is very small. The R cones are thus the only ones really sensitive to the red part of the spectrum, but their absorbance peak is located at the edge of the green part. To understand the link between these spectral responses and colored sensations, one must take into account the much more complex processing of these responses by the visual system. In particular, other cells compare the signals from the three types of cones (calculation of sums and differences) and also compare these signals between a given area of the retina and its periphery.

When we mix colored lights, we start by putting ourselves in the dark, the three types of sensors are inactive: it is “black”. Then we turn on a lamp emitting a light corresponding to one of the three primary “light” colors (red, green or blue), so we activate (in our simplified model) only one type of sensor. When the three lamps are lit, the three primary “light” colors are mixed so the three types of sensors are activated: we see white.

For the mixtures of colored materials it is the opposite: we start with a white sheet that reflects to our eyes the entire spectrum of light that illuminates it, the three types of sensors are activated: it is “white”. Then each primary “matter” color (yellow, cyan or magenta) deposited on the sheet deactivates a type of sensor. When the three primary “matter” colors are mixed, all the sensors are deactivated: we see “black”.

These two diagrams represent our simplified model of the rules of mixing light and matter (additive synthesis on the left, subtractive on the right), on which is based the logical reasoning that we can practice with young children. For each color “light” or “matter” are represented a spectral composition of the light (framed on a black background) and the simplified response of our visual system (blue, green and red traffic lights, which symbolize the activation of our three types of cones). In reality of course the situation is more complex: the precise form of the emission spectra of the RGB emitters used as primary for the mixtures of lights, as well as that of the reflection spectra of the pigments or CMY dyes used as primary for the mixtures of materials (without forgetting that of the source of white light which illuminates them) move away more or less from this model, and lead to a certain range of colors accessible by mixture.

Colored object and colored lights

The preceding diagram, which recapitulates the whole of the rules which we discovered by the experiment then interpreted thanks to a simplified model of the human trichromatic vision of the colors, is satisfactory but not necessarily very simple for a neophyte. It is good to reuse it in a slightly different situation, in order to consolidate its knowledge and its comprehension of the phenomena.

An experiment is therefore proposed to rework all the notions discussed during the training. It is a question of seeing and understanding how an object composed of simple colors is transformed when it is illuminated with colored light, or (which amounts to the same thing) when it is viewed through a colored filter. By reasoning about the way colored materials act on the spectral compositions of colored lights, we can understand the observed color transformations. Try it !

Through a red filter, we see the world in “red and black”, like here on this picture of a Rubik’s cube: yellow, white, red and orange squares become red, while green and blue boxes become black

Is it the end of the story?

To finish the training, we evoke colored phenomena which do not enter the simple framework discussed previously but which can give place to spectacular phenomena.

First example: all the colors that we meet around us do not belong to what is called chemical colors, that is to say the absorption by a dye or a pigment of a part of the light spectrum. In some cases, physical phenomena, such as interference, are at the origin of the modification of the spectrum of light: colors of soap bubbles, colored reflections on a CD, colors of polarization, etc.

Polarization colors are an example of physical colors: by placing strips of tape between two polarizing filters, beautiful colors appear depending on the thickness of the tape. By changing the orientation of the polarizing filters, the color of the background and the colors of the tape strips change (from one color to its complementary). The physical concepts involved are complex (polarization of light, birefringence of the tape, interference) but these observations arouse curiosity and wonder.

Second example: our visual system sometimes plays colorful tricks on us, which reflect its richness and complexity. Illustrations can be used to show the phenomenon of simultaneous contrast, which has inspired so many artists and which demonstrates that colors seen simultaneously interact with each other.

But the most spectacular example, which can give rise to very playful situations with children, is that of afterimages. By looking fixedly at a colored image, then by moving one’s gaze on a uniformly white background, one sees appearing (during a certain time) an image with the same geometry but with colors complementary to those of the original image (yellow becomes blue, cyan becomes red, magenta becomes green, and vice versa). This experiment shows that colors are sometimes in our eyes and in our brain, and not in the matter or the light we look at.

This arc is not in the colors of the rainbow: it contains magenta, and the order of the other colors is not right. If you stare at the small cross at the top of the arc for about ten seconds, then move your gaze to a white background (wall, …), you will see an arc with the colors of the rainbow (from top to bottom: red, yellow, green, cyan and blue) that will slowly fade over time.

Recommended references

As the training is devoted to French teachers, the recommended books and sites are in French. We thus suggest that you look at the original article.

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