The first thing I did was to try and find out from the audience members what their existing conceptions of lasers were, what images came to mind when I said the word 'laser.' Responses were usually along the lines of the standard science-fiction idea: a short bright line that flies from here to there and blows something up at the other end. 'Laser tag' came up several times, as did Star Wars--everyone seemed to know Han Solo and his blaster. One person responded, 'A red light,' which was interesting, although understandable, as most lasers we see around us are red. A couple of people had the correct idea, but they were a minority in each audience I presented to. I also had a laser set up (the larger and more impressive the better) and asked them to predict what would happen when I turned it on. Somewhat surprisingly, the entire high school audience seemed to believe that they would see a beam shoot from the laser to the wall, and perhaps cause some damage to the wall, while one member of a grade 5 class correctly predicted that they would see a bright spot on the wall, and nothing else. Most people thought they would be able to see the beam, and were surprised when they couldn't. I tried to get the audience to explain (with some assistance from me) why they couldn't see it, as well as suggest how we could see it (put something in the way to scatter the light). Following the suggestion, I shook some chalk dust into the path of the beam, which made the beam visible and looked very impressive, even with the lights turned on.
With this laser as an example, I talked about what the main differences between laser light and 'regular' light are. Depending on the level of the audience, I tried to get them to come up with some of the differences, and simply told them the ones they didn't think of. The main properties I was looking for, or that I emphasized, are that a laser is very intense, has a low divergence , is monochromatic, and is coherent, as mentioned above. Coherence I didn't explain to an elementary school audience unless they asked a question which required the concept for its answer. One approach that I considered but didn't get the chance to try out was to disagree more: have several examples of non-laser light sources that satisfy some of the characteristics the audience is likely to come up with, and say something like, 'Well, here is a light that has low divergence. Is it a laser?' This would require some preparation. Possible items would include the 'clunky pointer' or a directed flashlight, and a light with a coloured filter. They might need some help in the end, to finally distinguish between the two, as coherence is not a concept many students have encountered before. They might distinguish a laser from a filtered bulb by saying the laser is 'speckly,' or something like that, which is due to coherence (see below).
One distinction about the low divergence that I found worth making is that while it is possible to have conventional flashlights with low divergence, the object which emits the light in those cases emits in all directions (e.g. the filament in a light bulb), and it is only through lenses that the light is collected and focussed into a beam. By contrast, the light from a laser is only produced in a narrow beam. I emphasized this by taking apart the clunky pointer and showing its light bulb. This was also a good illustration of how bright the conventional bulb in the pointer had to be to even show up on the wall, whereas the laser showed up easily.
Also, to emphasize just how low the divergence of a laser is, I compared the laser to a flashlight with a narrow beam, such as the kind used for a bike headlight. I compared the increase in size of the laser spot from near the laser to across the room (from 1mm in diameter to roughly 1cm) with that of the flashlight spot (from 10cm to several metres). I also had one of the students measure the difference in brightness, with a small light meter, of the light at the two distances, which was quite dramatic: the brightness of the laser decreased almost none, while the flashlight was so dim by the time it reached the other side of the room that it almost didn't register on the meter.
I had a fish tank set up half-filled with water in which was dissolved a very small amount of cornstarch, which made the beam fairly visible when the lights were turned out. (It was more visible against a black background.) The top half of the aquarium I filled with smoke by burning incense in it for only about 20 seconds, and then put a cover on the aquarium to trap the smoke. The beam was quite nicely visible in both the smoke and the water, and by using a hand-held laser pointer, I could demonstrate refraction and total internal reflection very clearly. The fish tank was also useful for just making the beam visible in other demonstrations, such as comparing the laser to the beam from a flashlight.
I also did a small demonstration to combat the science-fiction misperception that lasers explode and damage things: I asked one member of the audience to hold out their hand and close their eyes. I then shone one of the lasers on their hand and asked if they could tell when I turned it on, which they couldn't. If a person can't even tell when the laser is shining on them, it isn't likely to do much damage! It can no more be felt than a flashlight beam can. Of course there do exist more powerful lasers that can cut through steel and rock, as I told the audience, but not among those I had at the demo. Indeed, most lasers are fairly harmless to a person, except to their eyes.
Usually at this point I would explain as much of the theory as I intended to, which depended on the age of the audience. To senior high school students I explained most of the concepts given in the Theory section above, with perhaps slightly less detail. To elementary school students I tended to give very little theory, for although I think they would have been able to understand most of it, I think I would have bored them. The Arbor Scientific acrylic-boxed laser was useful to refer to while explaining some of the theory, as it is very easy to see the laser tube with its electrodes and mirrors in this laser. I also passed the old laser tube around the audience so the students could look at it up close.
The acrylic-boxed laser also gives a nice demonstration of the way that, of the many electron transitions in the gas mixture, only one undergoes laser action. Looking at the laser tube discharge through one of the spectroscopes, it is possible to see the many spectral lines given off by the helium and the neon, which gives a good example of the energy levels discussed in the explanation. If the laser is shone onto a piece of paper and looked at through the spectroscope, it shows up as only a single red line, illustrating how, though there are photons of many (discrete) energies being emitted, only one energy (colour) undergoes laser action.
These aren't related as directly to what I was trying to explain, but they are good to show some other effects and keep the presentation more interesting.
I had two glasses filled with water, in which were dissolved small amounts of cornstarch and flour, respectively. When I shone a laser through them, the particles scattered the light and made the beam visible, as discussed above. In this case, though, the individual particles could be seen clearly, and the difference in size between the particles of flour and cornstarch was immediately visible. This would presumable work for suspensions of other materials with about the same particle size.
Two of the lasers I was using were the red helium-neon laser from Arbor Scientific and a green HeNe laser on loan from the UBC Physics department. If the spots from these two lasers were shone onto the same place on the wall, the spot looked yellow, which is an interesting demonstration of colour perception. (This idea could give rise to a whole series of fascinating experiments and demonstrations, that I don't have time to consider here.)
When a semiconductor laser is operated at a lower current than that required to create a population inversion, the energy of the electrons and holes recombining is given off as incoherent light, as in an LED. At a definite threshold current, there is a sudden transition between LED and laser operation. One of the demonstrations I used was a laser diode connected to a variable resistor, that permitted me to control the amount of current passing through the diode. Starting from somewhat below the threshold current and increasing it, the diode shone gradually more brightly until its output suddenly increased at the onset of laser activity. This is quite a dramatic demo. It is also interesting to note that the output of the laser is in a narrow stripe, reflecting the shape of the laser oscillator inside the diode.