Diffraction Limiting a Mk I Eyeball

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