On Jan 6, 8:25*am, Helpful person wrote:
On Jan 6, 3:23*am, "N_Cook" wrote:





Helpful person wrote in message

...
On Dec 30 2010, 8:06 am, Martin Brown |||

wrote:
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

You can also easily demonstrate the effect of aperture on MTF
(modulation transfer function). *Look at cubism type pictures or
pixelated pictures on TV (used to hide identity).

Using one eye, squint at the object. *This reduces the aperture,
cutting out the high frequencies and removing the sharp boundaries.
The "original" picture can be seen quite clearly.

www.richardfisher.com

&&&&&&&

But on the other hand if the light level is high enough and you've forgotten
your reading glases - you can read small print by "stopping down" an eye by
viewing through the central hole between bent-over thumb and first skin fold
of the index finger of a partial fist-making. Limiting to the central least
distorted part of the eyeball

It's not a matter of "less distorted" or aberrations it's a matter of
spatial filtering.

www.richardfisher.com

I know nothing of the science, but I have used this trick many times.
I use thumb, index, and social fingers to make a tiny triangle peep
hole. It allows near focus on objects almost in contact with the
cornea. I just tested it on my monitor, and I can see the 3 colored
bands (couldn't quite make out the individual pixels).