Date: Sat Aug 15 09:32:02 2009 Back to Contents ------------------------------------------------------------------------

Author: Urs Lauterburg

Subject: Re: FW: Why can't we see really small things?

Post:

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There are two physical parameters which limit the range we are
capable to detect electromagnetic waves (light). The visible range is
centered around the wavelengths at which the sun radiates the most.
The later aspect seems a very important one to me. It's the human eye
which adapted to the solar radiation and not vice versa. So in a way
the sun was one of our creators. Green is exactly where the plank
distribution of the sun's radiation has it's maximal radiation
intensity. Therefore it's not accidental that green marks the center
of the visible spectrum that spans from red to blue.

However, the two relevant limiting parameters are the wavelength and
the radiation intensity. The first defines the smallest object we can
perceive and the second sets the sensitivity threshold, thus the
lower limit beyond which the intensity is not strong enough to excite
the detector cones on the retina.

The wavelength defines the size of the smallest object by the fact
that at some point the wave properties of light begin to dominate and
from where the rules of geometric optics are no longer applicable. My
favorite way to demonstrate this is by showing the classic
diffraction experiment with a very intensive laser as a light source.
The effect is shown by slowly decreasing the width of a small
variable slit aperture which is passed by the laser beam. As long as
the aperture is considerably larger than a few wavelength of red HeNe
laser I use, the light beam just passes straight through the aperture
and can be seen as a bright red dot on a remote wall which acts as a
screen. Then if the width is narrowed and approaches the size of
several wavelengths the diffraction effects would gradually show up.
The light then forms the distinct diffraction pattern of minimal and
maximal local interferences. The separations of the maxima and minima
increase when closing the aperture even further. At one point the
pattern gets dimmer and then disappears altogether when the slit
aperture is completely shut. The demo is fundamental in the sense
that it shows the mechanism that determines the eye's resolution. The
effect actually defines the functional distance between the cones on
our retina. Again it's the eye which adapted to physics and not vice
versa.

The eye's resolution is defined under which angle we can still
distinguish two equally sized objects from each other. That's where
Dick's suggestion of car headlights comes in well.

The other aspect is how sensitive our eyes are to the radiation
intensity. The sensitivity is indeed incredible. In fact we should be
able to see a single burning candle from a distance of several
kilometers if we view it in a completely dark and undisturbed
environment. Of course such a place is hard to find these days on our
exaggeratedly civilized planet. But it explains why we see so many
stars during a very dark black night in a remote area.

To demonstrate the sensitivity quantitatively I actually measure and
plot the intensity pattern of a diffraction pattern formed by a slit
again. I scan the entire symmetric pattern with a photosensor that is
driven by a motor on a rail. The intensity is both linearly and
logarithmically plotted while the sensor slowly scans the diffraction
pattern. The chart is projected on top of the ongoing experiment. The
result clearly reveals that the intensity pattern we see with our
eyes corresponds much more to the logarithmic than to the linear
intensity distribution.

Needless to say that the range of electromagnetic waves goes from
radio waves all the way to high energy gamma rays. Light just covers
a very narrow domain in the broad spectrum. Physically it's all the
same, electromagnetic waves.

Well, I hope that my elaborations may give some ideas about some
aspects of the wonderful detectors which most of us use to navigate
through life. The eyes in combination with our brains make for an
incredible device.

I hope all the above will make sense.

Greetings from Switzerland

Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern
Switzerland


>Polly,
>
>I am forwarding your message to the PIRA community.
>
>Congratulations on discovering PIRA through your research. This
>organization is a unique global resource for creative educational
>schemes such as yours. I think you will receive some useful replies,
>
>Michael Thomason
>Director of Physics Learning Laboratories
>University of Colorado Boulder Department of Physics
> 303-492-7117
> thomason@colorado.edu
> http://physicslearning.colorado.edu
>
>From: Polly Billam [mailto:Polly.Billam@bbc.co.uk]
>Sent: Thursday, August 13, 2009 7:27 AM
>To: thomason@colorado.edu
>Subject: Why can't we see really small things?
>
>Dear PIRA,
>I'm writing from a major forthcoming BBC science production called
>'Invisible Worlds'.
>It's a flagship BBC1 documentary series (3x1hrs) made in conjunction
>with the Discovery Channel, due for prime time transmission next
>year, which is utilising a range of specialist camera equipment to
>capture the world ordinarily beyond the limits of the naked eye.
>Whilst the first two programmes cover 'invisible spectrum' stories
>and stories about extremely quick phenomena, the last programme is
>concentrating on the world beyond the resolution limits of the human
>eye (ie. the very small or the very remote).
>We are hoping to set up a simple demonstration (that could be done
>by our presenter, outdoors and on quite a large scale) of why it is
>that the human eye can't see very small things, based on the idea
>that the smallest thing that can be seen is one that subtends an
>angle at the eye that is subtended by one cone of the fovea (its
>image would just cover the receptor surface of one of the cones). I
>came across your work with the Harvard Natural Sciences Lecture
>Demonstrations online and noticed that you have conceived ways of
>demonstrating concepts such as resolution and wondered whether you
>had, or were aware of, or could suggest ways in which this might be
>achieved. The challenge is to create a visually impressive demo that
>would explain this idea so that an 8 year old might grasp it.
>PIRA have some great resources and expertise at their disposal. If
>there are any suggestions you are able to make or any way in which
>you are able to help I would be very grateful.
>Many thanks for your time,
>Best wishes,
>Polly
>
>Polly Billam
>Invisible Worlds
>Room MC5 D4 Media Centre
>201 Wood Lane, London W12 7TQ
>Tel: +44(0)20 8008 0502
>Mobile: +44(0)7968181288
>
>
>http://www.bbc.co.uk
>This e-mail (and any attachments) is confidential and may contain
>personal views which are not the views of the BBC unless
>specifically stated.
>If you have received it in error, please delete it from your system.
>Do not use, copy or disclose the information in any way nor act in
>reliance on it and notify the sender immediately.
>Please note that the BBC monitors e-mails sent or received.
>Further communication will signify your consent to this.

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Content-Transfer-Encoding: quoted-printable



There are two physical parameters which limit the range we are capable
to detect electromagnetic waves (light). The visible range is centered
around the wavelengths at which the sun radiates the most. The later
aspect seems a very important one to me. It's the human eye which
adapted to the solar radiation and not vice versa. So in a way the sun
was one of our creators. Green is exactly where the plank distribution
of the sun's radiation has it's maximal radiation intensity. Therefore
it's not accidental that green marks the center of the visible spectrum
that spans from red to blue.

However, the two relevant limiting parameters are the wavelength and the
radiation intensity. The first defines the smallest object we can
perceive and the second sets the sensitivity threshold, thus the lower
limit beyond which the intensity is not strong enough to excite the
detector cones on the retina.

The wavelength defines the size of the smallest object by the fact that
at some point the wave properties of light begin to dominate and from
where the rules of geometric optics are no longer applicable. My
favorite way to demonstrate this is by showing the classic diffraction
experiment with a very intensive laser as a light source. The effect is
shown by slowly decreasing the width of a small variable slit aperture
which is passed by the laser beam. As long as the aperture is
considerably larger than a few wavelength of red HeNe laser I use, the
light beam just passes straight through the aperture and can be seen as
a bright red dot on a remote wall which acts as a screen. Then if the
width is narrowed and approaches the size of several wavelengths the
diffraction effects would gradually show up. The light then forms the
distinct diffraction pattern of minimal and maximal local interferences.
The separations of the maxima and minima increase when closing the
aperture even further. At one point the pattern gets dimmer and then
disappears altogether when the slit aperture is completely shut. The
demo is fundamental in the sense that it shows the mechanism that
determines the eye's resolution. The effect actually defines the
functional distance between the cones on our retina. Again it's the eye
which adapted to physics and not vice versa.

The eye's resolution is defined under which angle we can still
distinguish two equally sized objects from each other. That's where
Dick's suggestion of car headlights comes in well.

The other aspect is how sensitive our eyes are to the radiation
intensity. The sensitivity is indeed incredible. In fact we should be
able to see a single burning candle from a distance of several
kilometers if we view it in a completely dark and undisturbed
environment. Of course such a place is hard to find these days on our
exaggeratedly civilized planet. But it explains why we see so many stars
during a very dark black night in a remote area.

To demonstrate the sensitivity quantitatively I actually measure and
plot the intensity pattern of a diffraction pattern formed by a slit
again. I scan the entire symmetric pattern with a photosensor that is
driven by a motor on a rail. The intensity is both linearly and
logarithmically plotted while the sensor slowly scans the diffraction
pattern. The chart is projected on top of the ongoing experiment. The
result clearly reveals that the intensity pattern we see with our eyes
corresponds much more to the logarithmic than to the linear intensity
distribution.

Needless to say that the range of electromagnetic waves goes from radio
waves all the way to high energy gamma rays. Light just covers a very
narrow domain in the broad spectrum. Physically it's all the same,
electromagnetic waves.

Well, I hope that my elaborations may give some ideas about some aspects
of the wonderful detectors which most of us use to navigate through
life. The eyes in combination with our brains make for an incredible device.


I hope all the above will make sense.

Greetings from Switzerland

Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern
Switzerland



Polly,





I am forwarding your message to the PIRA community.





Congratulations on discovering PIRA through your research. This
organization is a unique global resource for creative educational
schemes such as yours. I think you will receive some useful replies,





Michael Thomason


Director of Physics Learning Laboratories


University of Colorado Boulder Department of Physics


303-492-7117


thomason@colorado.edu <3D"mailto:thomason@colorado.edu">


http://physicslearning.colorado.edu
<3D"http://physicslearning.colorado.edu">





*From:* Polly Billam [mailto:Polly.Billam@bbc.co.uk]
*Sent:* Thursday, August 13, 2009 7:27 AM
*To:* thomason@colorado.edu
*Subject:* Why can't we see really small things?





Dear PIRA,


I'm writing from a major forthcoming BBC science production called
'Invisible Worlds'.
It=B9s a flagship BBC1 documentary series (3x1hrs) made in
conjunction with the Discovery Channel, due for prime time
transmission next year, which is utilising a range of specialist
camera equipment to capture the world ordinarily beyond the limits
of the naked eye.


Whilst the first two programmes cover 'invisible spectrum' stories
and stories about extremely quick phenomena, the last programme is
concentrating on the world beyond the resolution limits of the human
eye (ie. the very small or the very remote).


We are hoping to set up a simple demonstration (that could be done
by our presenter, outdoors and on quite a large scale) of why it is
that the human eye can=B9t see very small things, based on the idea
that the smallest thing that can be seen is one that subtends an
angle at the eye that is subtended by one cone of the fovea (its
image would just cover the receptor surface of one of the cones). I
came across your work with the Harvard Natural Sciences Lecture
Demonstrations online and noticed that you have conceived ways of
demonstrating concepts such as resolution and wondered whether you
had, or were aware of, or could suggest ways in which this might be
achieved. The challenge is to create a visually impressive demo that
would explain this idea/ so that an 8 year old might grasp it./


PIRA have some great resources and expertise at their disposal. If
there are any suggestions you are able to make or any way in which
you are able to help I would be very grateful.


Many thanks for your time,
Best wishes,
Polly





*Polly Billam*
/Invisible Worlds/
Room MC5 D4 Media Centre
201 Wood Lane, London W12 7TQ
Tel: +44(0)20 8008 0502
Mobile: +44(0)7968181288




http://www.bbc.co.uk <3D"http://www.bbc.co.uk">
This e-mail (and any attachments) is confidential and may contain
personal views which are not the views of the BBC unless
specifically stated.
If you have received it in error, please delete it from your system.
Do not use, copy or disclose the information in any way nor act in
reliance on it and notify the sender immediately.
Please note that the BBC monitors e-mails sent or received.
=46urther communication will signify your consent to this.




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From tap-l-owner@lists.ncsu.edu Sat Aug 15 09:32:02 2009

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