The Electromagnetic Spectrum VII - Visible Light Final Thoughts

Crossposted at Politicook.net.

Well, we have discussed the origin of visible light, its place in the electromagnetic spectrum, and some of the physiology of human vision. This installment will wrap up the visible part, and next we will consider the ultraviolet.

This time we will briefly consider the very different vision of most insects, compound vision, and finish up with a topic that almost everyone enjoys, fireworks.

Humans, other vertebrates, and some invertebrates have "simple" eyes, although the mechanism is extremely complex.  These eyes are called "simple" because each eye has only one lens that focuses a definite image upon the retina, making it possible to distinguish fine structure.  As an interesting aside, those species with eyes with just the right spacing can use the information encoded to triangulate the distance between themselves and the object that they observe.

Many birds can do this, and many others can not.  For instance, chickens have eyes almost on the opposite side of their heads, and so can see in 3D.  But they make up for that by being able to see in almost a 360 degree solid angle.  Hawks, on the other hand, have eyes positioned to the front and so have excellent spatial differentiation.  The difference is that chickens are prey, and need to see threats from everywhere, and hawks are predators and need to be able to "zoom in" onto potential prey.  I will leave it the reader to decide where humans fall on this divide.

Most insects have "complex" eyes, meaning that there are scores, hundreds, or thousands of small lenses that refract light from many angles towards their retinae.  Their eyesight is nothing like what we experience, because they see, on average, hundreds of blurry images.  But what it does do is give them extraordinary perception of motion, since as each lens is subjected to slightly different light exposure, the retinal image changes rapidly.  In addition, the spatial structure of this complex lens covers essentially 360 degrees.  This is why it is hard to catch a fly by hand, since they can sense movement very rapidly.

I promised fireworks, so here we go on them.  The term is generic for anything that gives off light (often with sound and/or motion) for entertainment purposes by chemical means.  All fireworks consist of a fuel, an oxidizer, and modifiers that determine the nature of the firework.

Let us start with the simple firecracker.  It is essentially a piece of thin paper into which has been rolled a burster mix, and fitted with a fuze (note that the term "fuze" is peculiar to pyrotechnics, and "fuse" means an electrical device to prevent circuit overloads).  The burster mix is typically a mixture of aluminum dust, an oxidizer, and some sulfur.  This is what we in the trade call a "hot" mix, because it is treacherous and very, very sensitive.  Hot mixes tend to ignite quickly and, if confined, explode.  The dangerous property of hot mixes is that their own weight can confine them enough for a disastrous explosion.

Then there are the middle mixes, which most colored fireworks use.  They are not as treacherous as hot mixes, but still must be treated with respect or they will kill you when you handle them.  Most colored mixes fall into this category.  Without going into a formulation diary here, the colors that you get have to do with the elements that are included, because the colors are derived from thermal excitation of electron to high levels, and what you see is the light emitted when those electrons "come back home" to the ground state (My apologies to Arthur Brown).

So, from red to blue, we have first Strontium, an element of Group IA, an alkaline earth metal.  These element have the advantage of being easily excited, so we will see more of them.  Almost all red fireworks use strontium as the color producing ingredient, and it is a very nice crimson, and not hard to make come out in a display.  There is a pretty wide margin for error with it.  It is not particularly toxic, so not much of a threat from it.  Just about any red firework is strontium based.

Orange is a bit more difficult, because the only common element with an orange emission is calcium, and its salts love to soak up water, ruining the firework unless kept in dryboxes that are too expensive for fireworks.  You will not see many, if any, orange displays.

Yellow is a bit easier, since sodium is a strong yellow emitter.  Some of its salts are not too hygroscopic, but still are not really good for fireworks.  Iron filings give a yellow, glittering effect, and are most often used for yellow, but it is from blackbody radiation, not discrete, quantized transations that make this possible.

Green in from barium salts, another group IA metal, closely related to strontium.  Barium is toxic, so it is not a good idea to sit downwind of a display.  But I do, because I love the smell in addition to the sights and the sounds.

There is another green, and it is hard to control, from copper.  More on that a bit later.

Blue used to be hard to do, and even arsenic was used to try to make a good one.  Modern fireworks use copper for blue and blue green, but it is tricky.  Copper itself is a weak green, but the copper chlorine molecular ion, CuCl+, is a fine blue if the temperature can be controlled in the device.  Too hot and it goes weak green, too cool and nothing.

The brilliant white displays are usually aluminum, which burns rich in the ultraviolet, but the illumination is from the microscopic pieces of aluminum oxide, heated up, as a blackbody radiator.

As an aside, modern fireworks are very recent.  Before about a hundred years ago or so, the only oxidizer available was potassium nitrate (saltpeter), which has a decomposition temperature too high for colors to appear, since it just ionized everything.  When potassium chlorate was discovered, everything changed.  It has a very low decomposition temperature, and so is perfect for colored fireworks.

The ones that celebrated the end of the Revolutionary War long ago were pretty much just bright, mostly white with some yellow, and loud, of course.  But none of the bright colors that we expect these days.

I will hang around for a bit for flames, questions, and comments.  Warmest regards, Doc.