At 15:50 14/07/2006, Andrew Larcombe wrote:
>A 1 metre wide object can be visible at 3400 metres to someone with 20/20
>vision. ... One example he gives is the suspension cables of the Golden
>Gate bridge which are less than 1 metre wide and in theory should be
>visible up to a maximum of 6880 metres in optimum favourable conditions.
>However, they are apparently visible from the campus of Berkeley, some
>20000 metres away, in sub-optimal conditions. Ogburn puts this down to two
>factors at play - the darkness of the cables against the sky, and the
>shape - ie a very long curvilinear feature.
This reminds me of some very elegant work done about 25 years ago in the
Health and Safety Executive laboratories on how fine asbestos fibres could
be before they were invisible under the optical microscope. At that time
this was a controversial question. Like the case mentioned by Andrew, it
was a question of the limiting visibility of a linear object and its
dependence on optical conditions. I have pasted below the reference and
abstract of the published paper.
American Industrial Hygiene Association Journal vol 43 pp 505-515
On the visibility of fibers by phase contrast microscopy
STEPHEN J. ROOKER , NICHOLAS P. VAUGHAN, JEAN M. Le GUEN
Abstract: The factors affecting the visibility of fibers under the phase
contrast microscope are discussed with reference to (a) the visual acuity
of the eye (b) the optical performance of the microscope and (c) the
optical properties of the prepared sample. For the first time, it has been
possible to measure the limits of detection of small fibers collected on
membrane filters. Full experimental methods are presented and the results
expressed in terms of phase shifts and minimum detectable brightness
differences between the fiber and the mounting medium filter background.
Fibers down to about 0.15Ám diameter are visible with good optics provided
the phase shift exceeds about 4░. This corresponds to a refractive index
difference of about 0.04, and provided this is exceeded further modest
changes in mountant refractive index make little difference to fiber count.