Purple is a strange color - one that you won't see in the rainbow - it doesn't have a wavelength associated with it.
I was trying to explain that color is something manufactured in the brain. There are a variety of ways you can do this, but the best is to project different colored lights on a white surface and let them overlap. Someone must have done this as a video ... indeed they have .. many of them in fact. This one shows the effect rather nicely and I sent it along.
The problem is the presenter makes an error about a half minute in in... He states that you can't mix photons. It turns out you can - in fact if you have ever seen a green or violet laser pointer you're seeing it.1 But explaining to a seventh grader who is curious about where purple comes from becomes extremely difficult and possibly impossible if I try to explain the what goes on with real physics.
What works is to ignore the rare case. It may be very interesting, but it isn't going to help the student. Mostly photons can't combine - it is a good approximation to assume that and use the result to explain color synthesis. This works throughout physics. To calculate the path of a baseball I can ignore relativity even though using just Newtonian physics is good enough. In fact I can ignore air resistance for some problems and get a good enough answer.
There is an art to knowing what to ignore and what to keep. You have to understand the size of the contribution, how good your measurements are and what sort of precision is required. The problem often arises in teaching. How much depth is appropriate and how much background can be assumed? There is a movement to remove jargon from the description of science hoping that the result will be interesting. Unfortunately you can only simply so much and that takes real skill - it is like writing poetry and I'm not terribly good at it.
As you learn more about a subject you develop a certain amount of experience to know what is easy, what is not and what is just silly. A friend happens to be one of the most famous computer scientists on the planet and his fame is quite justified. His email is easily available and he gets a lot from people who have "solved" fundamental problems that elude everyone else to those who have decided to do the next big thing and want something from him. He recently sent one that may be the best yet.2 It turns out he gets really wacky bits from people who lack experience. You may see this in your own field - I see it all the time in mine. One of the best experiences for me was being asked to look over a set of patents that managed to clear the USPTO despite describing a perpetual motion machine:-)
One thing I try to convince students of technology who are attempting to predict the future is to get their hands dirty and learn a bit about the fundamentals. Technologies don't spring onto the scene, but rather have an often lengthy development period. Their commercial use may combine bits and threads of other technologies or even art into something that strikes us as novel, but there are important early hints.3
I talked a client into trying to learn how to use a 3d printer and design and use one. This sort of exercise gives a much deeper view into what can be done as well as the necessary background for a user/designer. You are only getting a partial story if you aren't using the tech in question. She followed my advice and now has a much more realistic sense and timeline for the set of technologies.4
It is exciting that people are starting to learn how to design and build "stuff" these days. The notion of shop class 2.0 is on us. Building a personal shop and, more importantly, learning how to use the tools, is beyond most people. In a few cities Maker spaces have appeared - a concentration of tools, instructors, mentors, and other makers. You generally pay a monthly rental for tool use. This morning I caught a New Disruptors podcast where Glenn Fleishman interviewed a couple who have started MakerHaus in Seattle - Ellie and Mike Kemery. Recommended for anyone interested in one of the major "what next..." speculations - listen here (44 minute mp3)
I'm running low on time - there are dozens of ways this note could develop. I almost want to say something about the relative use of STEM and STEAM in organizations, but for another time.
In the meantime go out and get your hands dirty doing something.
1 The stuff of non-linear optics. This is way too complex to explain well here, but there is a really important safety issue involved and you have to be extremely careful with cheap non-red laser pointer. Green laser pointers are the most common as our eyes are very sensitive to green and we perceive the light as very bright. They are usually "diode pumped solid state frequency-doubled" and are sometimes called DPSS or DPSSFD lasers. A very powerful infrared diode laser is used to pump a crystal that lases at a somewhat longer wavelength. The light from the pumped laser is passed through a KTP crystal which acts as a frequency doubler - the wavelength goes from 1064nm to 532nm .. a very "green" light. The KTP crystal is a nice piece of non-linear optics where photons are effectively combined to produce one of a higher energy.
I don't know how to explain the lasing process and the non-linear optics without using some quantum mechanics. The problem is the pumping laser is very powerful and very dangerous - 100 to 300 mW! Looking at one of these would cause serious eye damage. To make matters worse the light is invisible to us. There is a secondary weaker, but still dangerous infrared beam .. either can cause damage! The cheap laser pointers are poorly designed and this light often leaks out. This is really bad - particularly in the hands of a kid.
There are lasers that emit in the green and blue and other colors directly that are generally much safer, but they are usually much more expensive. If you see a green or blue laser pointer in a meeting, I would ask if it has a safety certification and, specifically, a good infrared blocking filter. If not ask the presenter not to use it. I was handed one to use at a talk recently and it was clear it was a $40 special. I had to tell them to throw it away.
2 I've removed all names and other identifying information
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