Lasers and The Perfect Lesson

In my introductory physics class yesterday, I was talking about lasers. It proved to be one of the best classes that I’ve taught this term. Why? Because I only got through half of my prepared material. I was constantly being interrupted by the students asking me all sorts of good, relevant questions about applications of lasers. That’s the best possible lesson in my book. Maximum engagement, and a constant stream of questions from the students (preferably ones that I can answer immediately!). But it’s okay to be stumped by an answer, and tell the student “I’ll have to get back to you on that”. It shows that the teacher is human, and, provided that you deliver on your promise, that the student is valued and that dialogue is two-way.

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. This sounds intimidating, but the principle is quite simple. If a quantum system is in an energetic “excited” state (I always imagine a well caffeinated atom running frantically around), then if it encounters a photon (a particle of light) which has exactly the right wavelength, then it can drop down into a less excited state, and emit a second photon which is identical to the first and moving in the same direction.

So now you have two photons. If they both hit excited atoms, then you get four photons, then eight, then sixteen. All identical in wavelength and all moving in the same direction. Different quantum systems can produce different wavelengths, so all the colours in the visible spectrum are accessible, as well as infra red and ultra-violet lasers, which can’t be seen.

We use these for all sorts of things. Firstly, the laser moves in a straight line. So it’s great for surveying purposes. It can also be used as a range-finder, by sending a short pulse, bouncing it off a target, and timing how long the reflection takes to get back to the source, because we know what the speed of light is. This of course has military as well as civilian application, which might explain some of the fascination. Some of the questions were about military applications. It’s the social impact of the Death Star.

Wikimedia Commons

Since the laser can direct a very tight beam of light, which carries energy, it can be used for a variety of cutting, ablating and welding duties. There are many surgical techniques based on the use of lasers. My brother had laser eye surgery a while ago to reattach a detached retina to back of his eye. I always try to bring personal examples to the physics lesson.

One thing that I nearly always have a rant about is the lack of realism when portraying lasers in the movies. In the actual laser beam, all of the light is moving in the same direction. So unless you are viewing the beam head on (which is definitely not recommended at all) then you won’t see it. You can see it from the side, but only if there is something in the way which scatters some of the light. People using lasers in the lab sometimes have aerosol sprays which put fine droplets into the air, to provide that scattering medium.

So all those movies, where the criminal has to step over, or perform acrobatics to avoid all those criss-crossed laser beams to get to the valuable artwork/painting/atomic bomb? Nope, doesn’t happen. The smart criminal will bring along the aerosol can (hair spray would do) to reveal the beams, because invisible beams are a much better detection system.

My favourite laser application is using a big laser to bounce light off the moon, so that we can measure the distance very precisely, to a few centimetres, which over a distance of around 400,000 kilometres is pretty neat. For those poor unfortunates, oops, US citizens, not familiar with the metric system, call it an inch in 250,000 miles. And get with the rest of the world, use metric).

Photo: NASA. Apollo 11 Laser Reflector

This little reflector is enough (provided you have a stonking great laser on Earth) to reflect enough light back to Earth. We can measure the time for the round trip, and calculate the Earth to Moon distance.

You can see why this is a popular lesson, there is so much material to use. And we haven’t even got onto pushing things around with laser beams yet. So called “optical tweezers” are used in many applications. The photons in a laser beam carry momentum, and so when they hit objects can transfer momentum and before you can say “Newton’s Second Law”, then we are pushing things around. I usually show a video clip of a very cool real game of Tetris (You remember Tetris, don’t you?), but done with tiny glass microbeads being pushed around with lasers. And then you can mention DVDs and blue ray or CDs and the infra-red solid state lasers. It’s just too easy to come up with a good lesson plan, the trick is knowing what to omit. So that’s how to engage with a perfect lesson.

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