Tag Archives: phosphorescence

Why does Aurora Borealis occur?

One of the most spectacular phenomena in nature is without doubt the amazing games of light, shades and colors called Aurora Borealis and Australis, depending on which of the two poles it is perceived at.


Those who live at the extreme north and south of Earth might at times experience this colored spectacular lights shimmering across the night sky. But what makes these lights  appear?

Well, it may sound weird but everything begins from the sun.

The temperature above its surface is millions of degrees Celsius. At this temperature, collisions between gas molecules are frequent and explosive. Free electrons and protons are thrown from the sun’s atmosphere by its rotation and escape through holes in the magnetic field. Blown towards the earth by the solar wind, the charged particles get in contact first of all with our planet’s magnetic field which may be thought as been generated by a giant rectangular calamite positioned at the centre of the Earth. The structure of a rectangular calamite’s magnetic field is well known and is based on closed field lines getting out of the south pole and entering the north one. Exactly the same happens on Earth where we have to imagine a giant magnetic shield protecting the whole planet surface, except for the source (south pole) and the pit (north pole) of the field lines which are necessariauroraly more exposed.

The charged particles scattered all around by solar wind are largely deflected by the earth’s magnetic field. In particular these charges are trapped by the force of the magnetic field and they start following the force lines being channeled either towards the south or the north pole.  Therefore some particles enter the earth’s atmosphere and collide with gas atoms or molecules at various heights. These collisions  excite gas particles causing them to light up. Sounds something similar to phosphorescence… 

What does it mean for an atom to be excited? Atoms consist of a central nucleus and a surrounding cloud of electrons encircling the nucleus at increasing distances from the centre. When charged particles from the sun strike atoms in Earth’s atmosphere, electrons move to higher-energy orbits, further away from the nucleus. Then when an electron moves back to a lower-energy orbit, in order to lose the amount of energy it has gained, it releases a particle of light or photon. The color of emitted light depends on the atom and on the size of the inner electron’s jump, but the result is absolutely amazing as it involves billions and billions of particles emitting light at the same time.

aurora3What happens in an aurora is similar to what occurs in the neon lights we see on many business signs. Electricity is used to excite the atoms in the neon gas within the glass tubes of a neon sign. That’s why these signs give off their brilliant colors. The aurora works on the same principle – but at a far more vast scale.

The aurora often appears as curtains of lights, but they can also be arcs or spirals, often following lines of force in Earth’s magnetic field. Most are green in color but sometimes you’ll see a hint of pink, and strong displays might also have red, violet and white colors. The lights typically are seen in the far north – the nations bordering the Arctic Ocean – Canada and Alaska, Scandinavian countries, Iceland, Greenland and Russia. And of course, the lights have a counterpart at Earth’s south polar regions.

The most common auroral color, a pale yellowish-green, is produced by oxygen molecules located about 60 miles above the earth. Rare, all-red auroras are produced by high-altitude oxygen, at heights of up to 200 miles. Nitrogen produces blue or purplish-red aurora.

Several fascinating  myths and legends are connected to the phenomenon of auroras.

aurora2In Finnish, the name for the aurora borealis is “Revontulet”, which literally translated means “Fox Fires.” The name comes from an ancient Finnish myth, a beast fable, in which the lights were caused by a magical fox sweeping his tail across the snow spraying it up into the sky. The Lapps, or the Saami, a people who are a close relative ‘race’ of the Finns, who live in Lapland — that is, north of the Arctic Circle, in what officially are Northern Finland, Sweden, and Norway — traditionally believed that the lights were the energies of the souls of the departed. In Norwegian folklore, the lights were the spirits of old maids dancing in the sky and waving.  Several of the Eskimo tribes also connected the lights with dancing. Eskimos in Eastern Greenland attributed the northern lights to the spirits of children who died at birth.

 

That’s it! Cool, isn’t it?

GO BACK TO MR WHY!

by Francesco Pochetti

Why does phosphorescence occur?

Ever wondered why the little stars glued on our rooms’ ceilings go on glowing after we’ve switched off the light? What about funny shirts or glasses which are visible at night despite darkness? Why does all this stuff happen?

Well the phenomenon which is behind this cool events is called phosphorescence and we’are about to get a little bit of insight on it!


To better understand what goes on behind the scenes when this phenomenon occurs we have to ask ourselves a simple but fundamental question. What does it happen when a material is exposed to light? Which, in a more basic form, could be reasked in the following way:  what does  happen when a molecule is exposed to light?

Nice one!

fosfo1Without getting into a too detailed analysis of the events we could simply answer the question in this way: the considered molecule absorbs the incident light. Or better, considering that a light beam consists in “little energetic packages” called photons, we should say that when a beam of photons bombs a material’s surface, the inner molecules absorb light in the form of “energetic particles”. The primary consequence of this absorption is that the molecules which were hit increase their internal energy. This energy, however, cannot be kept forever by the molecular system. In general it is quite immediately released by the molecule. This phenomenon is extremely fast and we could never appreciate it to the naked eye!

But let’s see a little bit more in detail what happens inside the molecule right after a photon absorption. There is a huge amount of extremely complex phenomena which are triggered by the absorption of light; all of them can only be explained using quantum mechanics.

fosfo3Nevertheless it is still possible to have an idea of what’s going on in the following way. We first have to accept that each molecule has only well defined accessible energy levels, which means that everything hitting the system won’t automatically be absorbed. We can imagine the reachable energy levels of a molecular system as a building’s several floors. We have also to imagine that these floors are connected one to the other by an internal lift, which lets us reach them from the bottom to the top, and that, in the meantime, we can only use the stairs to go down. That’s it? Absolutely not! There’s another complication. While going down we cannot necessarily access to each floors, as if we had a direct access from the fourth floor to the first one but in order to pass from the third to the second we found a closed keyless door. Forbidden transition there!

Our molecule can be compared to a young man living on the ground floor of this imaginary building and our absorbed photon as a sort of nutritional supplement giving the weak young man some energy to stand up and climb the building to higher floors!

Ok.. So, what does happen after a molecule has absorbed (the right amount of) energy?

Our young man can now stand up and, completely revitalized, takes the lift till the floor allowed by the amount of acquired energy. That’s exactly (more or less!) what happens to a molecular system. It absorbs a photon whose energy excites the molecule to defined level. And what about the energy release?

Our young man has to descend back to the ground floor in order to lose all he has acquired. That’s not easy at all because there is the probability for him to find a forbidden path from a floor to an other. A closed door. What then? Theoretically he should stop and stay there, hopeless. Practically he could, for instance, force the door and access the forbidden transition. Obviously it would not be so fast at all. He would need time to open a passage and finally crash the door. Probably plenty of time. But finally he would succeed and he’d be able to go back to where he began. The ground floor!

After this awesome little story we are able to answer the first real question. Ever wondered why the little stars glued on our rooms’ ceilings go on glowing after we’ve switched off the light?

fosfo2Here’s the answer: when we turn the light on, the stars begin absorbing energy. Or better the molecules inside the material start absorbing photons and get excited to a well defined molecular energy level. Immediately after they try to release this energy but it may happen that the system, attempting to go back to the ground floor, finds itself stuck at a particular energy level. Quantum mechanics reads that, in theory, there are some forbidden transitions. No way to pass! In practice, however, the molecule succeeds in forcing its passage to a lower molecular energetic level. Generally, this operation requires plenty of time, which means that our little star on the ceiling goes on glowing for minutes or hours after its first absorption.

When we switch off the room’s light we will be able to clearly see the phosphorescent star which is slowly releasing the absorbed energy  in the form of light! Phosphorescence! That’s it!

Cool isn’t it?

GO BACK TO MR WHY!

by Francesco Pochetti