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LaserScience
Backgrounder for teachers
Persistence of Vision
- Light takes a fraction of a second to fade in human vision. If a spot of light moves quickly, it leaves a fading trail in the vision of the viewer.
- The brighter the light, the longer it lasts in the eye, allowing people to “write” with sparklers or lasers or flashlights. (Sparklers are very bright, which makes them easy to write with, but they can be too bright for some people’s eyes. Do not stare at them.)
- Laser Shows only need a single narrow beam, projecting a small dot, to project any image. It’s simply a matter of moving that dot around faster than its trail fades in the viewers’ eyes.
- A movie screen goes dark 24 times per second while the projector switches pictures. Because of persistence of vision, the earlier picture is still fading in our eyes, so we don’t notice the darkness at all. Televisions and Computers (but not the new LCD displays) “flicker” off and on between 25 and 100 times per second. While persistence of vision does make the movie appear to stay “bright” there is debate whether that is what makes us see motion as well.
Inertia
- Light, like any object, has inertia; it moves in a straight line until something reflects it. In fact, if an object did not reflect or bend light into our eyes (or block us from seeing light we were expecting to see), we would not see that object at all!
- With nothing to reflect the light, neither our laser beams nor the light from a flashlight is visible until it has hit an object.
- The reason toy “light sabers” need the plastic cone atop a light is to reflect the light to viewers’ eyes. Spraying droplets of water in the air will accomplish the same goal.
- Try this together: Spray a very fine water mist in the air (a spray bottle set to “mist” should work fine) and while spraying, shine a flashlight through the mist.
- A similar effect can be achieved with light through fog. During the program we “surround the audience with billions of tiny reflectors” to bounce light into students’ eyes. This is a hypoallergenic “haze” designed for this purpose. It is made by spraying a water-based solution at a very hot needle, which quickly boils and condenses it into minute droplets which take a long time to evaporate. A fan blows them into the air.
Energy Conversion
- Atoms and molecules may absorb energy, usually trapped in the motion of their electrons
- That energy is not trapped for long, and when the electrons “settle down” atoms often release that energy as light, although we don’t always see it.
- Electricity, heat, and light can energize atoms to release light. We will demonstrate all of these.
- Humans conduct electricity, allowing electrons to flow through them, although not as well as wires. This is why we can be electrocuted. In demonstrating the conversion of electricity into light, we will use a low-power, high voltage electricity source and pass electricity “through” (actually over the surface of their skin) students into light bulbs.
LASER: an acronym for Light Amplification (making more/bigger) by Stimulated (atoms stimulate each other) Emission (giving off) of Radiation (light)
- Atoms’ electrons can only release light of certain colors (frequencies). Each different chemical has its own set of signature colors. Neon’s are pink, mercury’s are blue, etc…
- Einstein predicted that if an atom is storing energy, and gets hit with light of exactly the same color that that atom can release, then it will release its energy in a “follow the leader” game of sorts – it releases the same kind of light, in the same direction, at almost the same time.
- If a lot of atoms are all charged up then a pulse of light can stimulate one atom, which may stimulate two more, and two more, until an “avalanche of light” might build up.
- Put those atoms in a narrow tube, with a mirror at both ends, and the avalanche will build back and forth in the tube. If we continue to re-charge the atoms, the power of that avalanche builds.
- If one mirror lets a bit of the light through (like the two-way interrogation mirrors in cop shows) what emerges is… a LASER beam.
- A LASER is simply a tube of molecules (usually gasses like helium and neon) with a mirror at each end, with a power source that repeatedly energizes atoms.