"N_Cook" wrote in message
...
| Martin Brown wrote in message
| ...
| I was doing a Christmas lecture earlier in the the week on "Light and
| Colour". I had an unexpectedly strong positive audience reaction from a
| very simple and cheap demonstration using 0.1-0.2mm diameter pinholes
| in aluminium foil held to the eye. Materials are so cheap and prep so
| easy that everyone can see this with their own eyes. And it is far more
| convincing to them than any number of fancy laser based demos!
|
| This is roughly equivalent to having a pupil stopped down to 1/10 or
| 1/20th of the normal 2mm daylight size and is small enough to give a
| very clear stable Airy disk on point sources like Christmas tree lights
| whilst still retaining colour vision. The bullseye appearance of point
| sources came as a big surprise to those without a science background.
| Edges are obviously soft and bright areas surrounded by a darker halo.
|
| The main light source was a desktop quartz halogen with foil hung over
| the front and a 2mm hole in line with the bulb filament. This gave very
| pleasing views of the diffraction pattern of the pinhole. But it was the
| coloured Christmas lights that people found most striking.
|
| And by swapping pinholes (which were of somewhat variable size) with
| their neighbours the audience quickly realised for themselves that the
| smallest pinholes had the biggest and faintest diffraction patterns.
|
| It also increases depth of field allowing very fine print to be read
| with relative ease when held suitably close to the eye. I discovered in
| the process that a few young children in the audience with otherwise
| normal vision could read 2pt text with the unaided eye!
|
| I hope this is of use to others in showing convincing diffraction
| effects to a lay audience using minimal cheap hardware.
|
| NB the real Royal Institution Christmas science lectures are titled
| "Size Matters" and are being televised on BBC4 at 8pm this week.
|
| Regards,
| Martin Brown
|
|
| An umbrella and nightime streetlamp is inexpensive for diffraction
pattern.
|
Take an ordinary CD or DVD and hold it horizontally, the rim touching
your cheek just below your eye. Now look at a street light, in the CD.

I took this image of a fluorescent lamp using a web cam.
http://www.androcles01.pwp.blueyonde...pDVDWebCam.JPG


| I've taken over running the Southampton part of the "Science Cafe" lecture
| structure if anyone in central Southern England could give some sort of
| science talk in
| 2011
| http://www.divdev.fsnet.co.uk/scicaf.htm
| national structure
| http://www.cafescientifique.org
|
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