White Super Paint, Black Cats, and Colors We Don't Really See

"Big Blue Marble", NASA photo from Apollo 17

 As we transition back into a semblance of normality from the pandemic lockdown, we found ourselves with a few mismatched pieces that didn't quite fit in previous posts, some updates from recent news articles, and a few new questions from readers that we wanted to deal with before we launch several new topic threads in this blog. So, we've presented some of them here in the format of one of our seasonal Q & A posts.

 — J.D.

 

You mentioned a white paint that was somehow "super" in the Psychology of Color article. Can you tell us more about that?

Yes. Engineers at Purdue University have come up with an ultra-white paint. The process took nearly six years. The paint reflects approximately 98 % of sunlight, which has a major cooling effect. The idea is not new. People who live in very hot countries have used light-colored building materials or very white paints to cool their homes for centuries.  The engineers used a compound of barium and sulfur. This isn't new either.  The same compound is used in making photographic paper and in some cosmetics.

Painting roofs with this paint could cool houses and cars more efficiently than any type of air conditioning — and at far less cost. If used on a large scale on roofs and the tops of vehicles, perhaps even on ships,  total energy use could be reduced markedly.  Reduced energy use would reduce climate change over large portions of the planet.

Do you know that I had to look up more words in the dictionary to understand your article on The Psychology of Color than I have had to do for whole chapters of my high school textbooks?  I've got pages and pages of notes!!!! I don't know if I will ever use some of them but I will know them if I see them again!!!! Whew! Should I thank you or what?

No need to thank me. You did all the work. I am impressed and extremely proud of you. I hope you are equally as proud of yourself.  I had a group of girls in one of my classes who decided to learn a new word every day. They would get together at lunch to exchange lists.  By the end of the year they had fat notebooks, too, but they never attempted to do as much in as short a time as you have done — and it was a collective effort. May I suggest one more addition to your notebook? Write a sentence for each word that is not the same as the sentence you may have found in your dictionary, but that is just as accurate. Then you will truly have command of that word. Congratulations!

You showed a picture of a black cat but didn't talk about it. How did it happen that people thought it was bad luck if a black cat crossed your path? Do you know?

That is a long and complicated story, but I will try to give you a condensed version.  Cats, especially black ones, were revered as long ago as 3000 B.C. (BCE). They were often mummified after they died and placed in tombs. They kept the mice and rats out of food storage and were good companions. To kill one was a death sentence. The Japanese also held cats in high esteem.  

Then came the Middle Ages in Europe (500 to 1500 AD). a time when literacy wasn't widespread and superstitions flourished.  The Spanish appear to be the first to associate black cats with witches.  The Irish, Welsh, and Scots didn't seem to take the idea seriously, but the English and Germanic tribes did.  It is an English story that seems to have done the damage. It goes like this: a man and his son were walking along a path one evening when a black cat appeared on the same path. They threw rocks at it to drive it away and struck it hard on one leg. It ran into the house of a lone woman whom some people thought practiced witchcraft. The next day the man and his son saw the woman, who was limping. These two "geniuses" then told everybody that the limp was from a rock they threw at her the night before when she was in the body of a black cat. The story spread like wildfire and became the cause of the idea that it was bad luck to have a black cat cross your path.   During the long pandemic of Bubonic Plague, people killed thousands of black cats. Now that caused REALLY bad luck because the plague was carried by mice and rats, which those cats could have killed and kept the plague from spreading! The Europeans brought their superstitions to America, causing such disasters as the Salem Witch Trials, in which 200 people were accused of witchcraft — and 19 of them were hanged.

There is a story going around that it is difficult to get people to adopt black cats from shelters, but the statistics don't support it.  Of all the many cats my family has lived with, more were black than any other color.  A word of caution, though.  Black cats are still injured and killed at Halloween, so it is a good idea to keep yours safely indoors at that time.  Enjoy your black cats!

 

Some time ago, in the very first issue of this series on Color, we briefly discussed structural color and gave an example or two.  We discussed the fact that some of the colors we think we see are not really there. They are illusions produced by complex layers of structures. Since one of the colors we discussed in Part I of The Psychology of Color was Blue, this might be a good time to warn you that most of the things that we think we see as blue are products of structural color.  The sunny blue sky we see is one of them.  So is the deep blue of the ocean.  And the feathers of the bluebird are not really blue.  Which brings up several questions that I will try to answer briefly. (If you are really interested, there are many discussions of these topics on line.)

Why does the sky look blue to us in the daytime when outer space looks black?

The earth has a thick cover of atmosphere that has been forming for millions of years.  The light from the sun hits molecules of various gases in that atmosphere and they fly off in all directions. The blue color of the sky is the result of this scattering. Because the Earth rotates,  the light changes as the planet turns away from the Sun each day, leaving the sky black at night. If you were on the moon, which doesn't have an atmosphere, the sky would look black both day and night.  Pictures taken by early astronauts show that very clearly.
 
Why does the ocean look blue when water doesn't really have much color?

According to NOAA (National Oceanic and Aimospheric Administration), it gets its color because of the way it absorbs sunlight.  It strongly absorbs long wavelength rays of the color spectrum as well as short wavelength ones, even into the ultraviolet range.  The remaining light that we see reflected from the ocean is mainly made of blue wavelengths.  Objects floating just under the surface, such as algae and mats of seaweed,  often give the ocean a green hue.  If you dive below the surface and continue down the water column, the water becomes darker and darker until all color disappears into the darkness.

Why does a feather look blue if it doesn't have any blue pigment in it?

Hyacinth macaw (public domain)

If you hold a feather of any bright color that does not have any pigment in it up to a light, it loses its color and becomes more or less white (sometimes a slightly dirty-looking white).  It becomes obvious that the color you originally saw on the feather was structural color. Move the feather back and look at it in normal light, and its color reappears.  The structure of the feather itself determines what color it appears without pigment.  Feathers that appear blue are covered with tiny air pockets that reflect short blue wavelengths.  These pockets are actually filled with liquid when the feathers form, but the liquid is quickly replaced by air.  If the pockets are arranged in a very regular way, the feathers will be a consistent color of blue like the wings of a macaw.  If the arrangements are not quite regular, the feathers may show several different blues. (Think of a blue jay's feathers.)

There are very few animals that are truly blue.  However, if you watch for our Frogs for Fun issue this summer, we will introduce you to a true blue one.

Do something colorful!

 

This post by Annake's Garden is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

Spectral Light and Structural Light: All About Color, Part I

Sunset on the ocean   (photo by J.J.)

Light reaches the Earth from the Sun in several different forms. Two of the most important — infrared and ultraviolet — are not visible to us without the use of instruments. There are living things that are conscious of some of those wavelengths, but we are not among them. In between infrared and ultraviolet there is a narrow band of visible light which contains the wavelengths of all the colors that we can see with the naked eye. We call this "white light".  In this case, white is not a color; rather it is the presence of all colors. The black of interstellar space, also not a color, is the absence of all visible light. When a light, natural or artificial, appears, so do colors. When there is no light, there are no colors. We are all familiar with the phenomenon that occurs when a beam of visible light passes through a glass prism and produces the spectrum of color that we associate with a rainbow.

White light through a prism  (public domain graphic)

Colored objects

But when we talk about the colors of objects — paper, paint, cloth, wood, plastic — white is the absence of color and black is the presence of all colors. When elementary school children hear that, they immediately want to mix all the colors in their paint box to try to make black. But the best they can achieve is a muddy brown. It is a "teachable moment"*.  Light is energy; pigments are matter. They follow different rules.

Peacock plume

Just to make the situation more confusing, many of the colors we think we see are structural colors, physical illusions, tricks of the light. They are no more tangible than the rainbows we discussed in the "Skyscapes" series (June 2, 2019 post), but they are everywhere in nature — a butterfly's wing, a peacock's feather, a fish's scales, the blue of the sky. Structural color depends on light, not pigment.

Clear blue sky, showing color gradient

The atmosphere is filed with dust particles and very tiny droplets of water. These are comparable in size to the shortest wavelengths of visible light. That is the blue/violet range. These wavelengths are small enough to enter droplets and pass through them, being bent both at entry and exit, so they are scattered and reflected, while the longer wavelengths of light — red, orange, yellow, and some greens — pass through the atmosphere intact. (Rainbows, of course, depend on much larger raindrops.) Lacking intervening clouds, we see the sky as shades of blue. If it were not for the atmosphere that surrounds our planet, the sky would be the black of outer space. The way all this works was discovered and described by a physicist, John Tyndall, in the middle of the 19th Century. It is called the Tyndall effect. But artist Leonardo da Vinci had written about the phenomenon in his notebooks much earlier, noting that the air appears more blue the further away it is from the viewer. He correctly theorized that this was because of moisture in the air.

We followed a Morpho butterfly for nearly an hour through mature rain forest in Panama. (Mature rain forest is fairly open and easy to travel through, not like the "jungle", which is the result of damage and disturbance.) The butterfly, whose wings were as broad as my two hands pushed thumb-to-thumb, flew slowly and majestically from one food source to another. Flashes of sunlight reflected from its brilliant blue wings, which you can see in the accompanying photograph. But the blue was not in the scales on its wings, which are tiny and clear. The wings of the butterfly beneath the scales are a warm, dark brown. These absorb the longer wavelengths of light, while the scales — like the droplets of water in the air — reflect the short blue waves of visible light. This structural light is subtractive;  that is, it depends on the array of wavelengths of light that pass through, too large to be taken in and reflected or refracted.

Morpho butterfly  (public domain photo)

RGB Additive Color Space  (public domain graphic)

We learned early in life that there were three primary colors — red. yellow, and blue — and that we could mix those colors of paints or crayons to make the secondary colors — orange, green, and purple.  A few years later we learned in a science class that light came in different primary colors. These were cyan (a light greenish/bluish color),  magenta  (a bright reddish/ purplish color), and yellow (we knew that one). This distinction goes back to Sir Isaac Newton. Scientists have learned quite a bit about light and color since Sir Isaac's time. They had to do so in order to make technicolor movies, color television (yes, children, both of those used to be in black-and-white), and the display on whatever digital device you are using to read this post. These broadcast on the principle that visible light is made up of red, blue, and  green wavelengths. (This made sense to me because green wavelengths are located about half-way between red and blue wavelengths.) If you overlap these colors of light, red and green make yellow, blue and green create cyan, and red and blue create magenta. The three primaries, combined in different proportions, give us all the colors we see on our screens — and in the world we see when we look up from them. (The primary colors of light are the three colors of light which, together, produce white light. Mix any two primary colors and you get secondary colors. Mix a secondary color with either primary in its make-up and you get a tertiary color.)

Materials for cellophane experiments

If you want to see some of the color-mixing effects for yourself, look for shops that carry gift-wrapping paper. Buy a roll each of red, yellow, green, and blue cellophane. Then all you need is a sunny window and a piece of white material to tape to the wall opposite the window. Do either the RBY or the RGB overlapping arrangements give you the results you expected? (If you are working with children and want to let them experiment with overlapping colors, it is a good idea to make frames for the cellophane. Chose a salad plate and a saucer that is at least a half-inch smaller all around than the plate. Draw around the salad plate to make circles on white poster-board.  Center the saucer on each circle and draw around it. Cut out the center of each frame. Smooth a piece of cellophane flat on a table. Use white glue or rubber cement on the bottom surface of the frame and place it on the cellophane. When the combination has dried, cut away any surplus cellophane from the outer edge of the frame.

An atom  ( public domain graphic)

When light strikes an object, one of several things can happen. We mentioned earlier that light is a form of energy.  The smallest unit of light is called a photon. Any material that the photon strikes is made up of atoms. An atom has a center called the nucleus, surrounded by orbiting electrons at several different levels, but most of the atom — much like our solar system — is empty space.  I mean a lot of space!   Think of Yankee Stadium as an atom. The nucleus is about the size of a small gumball at the center of the playing field.  The electrons are all racing around in the last rows of the cheap seats, all the way up on the rim!

Ways light can interact with matter

When a photon strikes an electron, one of several things may happen.  If the photon gives up its energy to the electron, the photon disappears. We say the light has been absorbed.  If the photon is just slowed down by the electrons of a substance, it is bent. We say it is refracted.  Shiny, smooth substances tend to bounce some photons back toward their source. We say the light is reflected.  Sometimes the light passes through the substance and comes out the other side. We say the light has been transmitted.  Sometimes the photons seem to bounce in all directions at once. We say the light is scattered.  All these statements are over-simplifications, but they express the general ideas.

CMY+K  Subtractive Color Space  (public domain graphic)

Under the previous system (Newton’s), yellow and cyan create green, magenta and yellow make a red-orange, and magenta and cyan produce a dark blue-violet. Those are not large differences, but they are significant, especially when you begin working with secondary and tertiary colors. But there is one very important art enterprise which not only depends on the cyan/yellow/magenta model, but also adds true black to the mix. This industry is color printing.

Essentially this means what we see on our digital screen does not give us an accurate assessment of what is going to emerge from a color printer. Since J.D. deals with this problem all the time, I'm going to turn the discussion over to him.

J.D.: I’ll bet you have never seen a printer that prints white lines on black paper, although building one would be fairly simple. The problem is that your white ink would have to be completely opaque — any black paper showing through would turn your white lines noticeably gray. But thirty years ago, printers were made to do much the same thing: they only printed opaque black lines (words) on white paper.

However, to get more than two colors — one from the ink, one from the paper — with opaque inks you need at least one container of each basic color (let’s call them ‘pots’), plus a third to mix them in; then you need a way to mix them in the right proportion (to keep it simple, just think of it as a magic elf). Plus, if you need to mix the inks more than once, or change the proportion of colors in the mix, you need a way to thoroughly clean the mixing pot (add another elf) or another new pot. And, with every color you add … well, even with teeny tiny magical pots and an army of magic elves, it is easy to imagine your printer soon becoming the size of a factory, and the time you have to wait for your print job stretching to weeks.

The solution printer manufacturers found was to use inks that weren’t opaque, called dyes. These work like the colored cellophane that Annake discussed earlier: they lay one small dot of see-through color over another on the paper until they get the mixed color you ask for. This eliminates a lot of the pots of colors you need, including all the mixing pots (that happens right on the paper), puts most of the magic elves out of a job, and can be done a lot quicker.

This solution is pretty good, but not perfect. For one thing, like the children mixing paints mentioned above, you can’t get a good black this way. But since the printer manufacturers already had a good opaque black cartridge, they just added it to the machines along with the dye colors. However, you can’t mix the opaque black ink with the colored dyes, and there is no way to thin it to make it see-through. And that is why, when you tell your printer to print in black and white, it uses only the black cartridge; but, when you specify shades of gray (gray-scale), it only uses the color cartridges.

Blown glass 'balloons'  (public domain photo)

Annake: Finally, the window in my office is made of glass. The glass, which is in a double layer, is set in metal frames. The frames are inset in a wooden wall, which is covered on the inside with plaster and on the outside with shingles. All of these materials are solids.  Light does not pass through metal. wood, plaster or shingles. Why, then, does light pass through solid glass?  Glass is an amorphous solid. Its atoms do not form structure in an orderly fashion like those of metal or wood.  The atoms of glass arrange themselves in a random fashion, much like the atoms of a liquid that flows do.  So glass is stable and rigid like a solid, but has a disordered arrangement of atoms like a clear liquid. Therefore, light can pass through it. At least part of the reason for this is that the silica sand used in making glass is heated to extremely high temperatures and then quickly quenched  — plunged into a cold liquid — to harden it. Perhaps the glass atoms simply don't have time to form a more organized structure.

Glass is a fascinating material.  If you ever have the chance to watch glass-blowers or stained-glass artists at work, please spend some time with them and take along some children to see these artists at work.  The children will enjoy and remember watching them as they ply their ancient arts.

Enjoy color in all its forms!

*If you are a parent, a teacher, or anyone working with children, you can find additional material for activities you can use for this “teachable moment” in the new ‘Extras’ tab in the tab menu at the top of this and all future posts!

"Recycled Rainbow", crocheted afghan by Annake

All illustrations by J.D., unless otherwise credited
 This post by Annake's Garden is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.