Re: [Phys-l] index of refraction

[John Denker <jsd@av8n.com>]

On 05/13/2009 06:03 AM, Paul Lulai wrote:
> The lab was phrased to make it an application / lab problem.  I told
> them we were a small lab attempting solve problem xyz for boss abc.
> The boss wants to know the width of some small item, maybe the width
> of a wire, hair, or something else.  Our job is to see it this
> wavelength of light is appropriate for the job.  Wavelength affects
> resolution.  Turn out our lab has some old lasers that haven't been
> used and documentation is lost.  Find the wavelength of our existing
> lasers so we or our boss or your employees know what sort of
> resolution we can get from the use of these lasers.

I understand where that is trying to go, but it falls off the rails 
before it gets there.  Some constructive suggestions appear below.

I always thought that the main point of "rich context" problems is 
that the students are supposed to rack their brains and think of little 
bits of real-world information that they can bring to bear.  In this 
example, such info weakens the scenario rather than enriching it.

In particular:

  I can determine the diameter of a hair within a few percent
  using a micrometer caliper ($20.00 at Harbor Freight).

    IMHO learning about micrometer calipers is at least as important
    and more age-appropriate than learning about lasers.

  The wavelength of light is not important for visually observing 
  objects the size of a hair.  The scenario is restricted to the 
  visual spectrum, which has a narrow wavelength range to begin with.

  You don't need a laser;  incoherent white light works fine.
  Students ought to know this already, having looked at hairs,
  red blood cells, etc. in biology class.

If you want to motivate the idea of calibrating a laser wavelength,
you don't need to fuss with hairs or old lasers in the attic.  An
ordinary cat laser from the dollar store has an undocumented (and
irreproducible) wavelength.  You can calibrate it using a CD as a
diffraction grating.  If you don't know the track pitch of your CD
you can calibrate that using any standard of wavelength, e.g.

  Ye olde HeNe laser is 632.816 nm.  No documentation required.

  As for my green "pocket" laser, I know immediately that it is 
  525 ± 25 nm because it is _green_.  In fact it has to be 532 nm, 
  since it's a frequency-doubled Nd:YAG.

  Or sodium D-line.