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Subject: Re: You use different lenses underwater because the speed of light is different




"obt" wrote in message
news:1hei6t0.p8ekho18s5fd9N%here@now.au...
> William Graham wrote:
>
> > "Prometheus" wrote in message
>
> > >
> > > Of course there is no water inside the lens (or should not be) however
> > > there is water against the front of the lens and if a ray of light
> > > reaching that front surface is not perpendicular to it then the angle
of
> > > refraction on entering the lens will be different between air and
water.
> > > Refraction can be considered as group delay across the interface,
whereas
> > > chromatic aberration is a result of phase delay, the former modifies
focal
> > > distance, the latter chromatic aberration.
> > >
> > Light travels about 75% as fast in water as it does in air, so any lens
> > designed for air use will have an incorrect focus scale when used
> > underwater....Light from infinity will focus behind the film plane when
the
> > lens is used underwater. but this effect will go away if water is
prevented
> > from contacting the front lens element by either a filter or an
underwater
> > housing, which would be necessary in any case if the lens was not
designed
> > for underwater use in the first place, so I wouldn't worry about it.
Unless
> > I was doing (and being paid for) professional underwater work, I would
just
> > buy a Nikonos with a decent 35 mm lens, and use that for my underwater
> > pics....I wouldn't trust my F5 in one of those plastic underwater
housings
> > when you can buy a Nikonos with a good lens for under $500.........
>
> Honestly, I thought you guys are all joking.
> To sumurise theory of relativity, the speed of light is the universal
> constant, it never changes, through vacuum, air or water. It refracts,
> just no speed bumps...

Then how are folks able to stop it?
http://www.nasa.gov/home/hqnews/2003/may/HQ_news_03176.html


In their laboratory, Hau and her colleagues have been able to slow a pulse
of light, and even stop it, for several-thousandths of a second. They've
also created a roadblock for light, where they can shorten a light pulse by
factors of a billion.

"This could open up a whole new way to use light, doing things we could only
imagine before," Hau said. "Until now, many technologies have been limited
by the speed at which light travels."

The speed of light is approximately 186,000 miles per second (670 million
miles per hour). Some substances, like water and diamonds, can slow light to
a limited extent. More drastic techniques are needed to dramatically reduce
the speed of light. Hau's team accomplished "light magic" by laser-cooling a
cigar-shaped cloud of sodium atoms to one-billionth of a degree above
absolute zero, the point where scientists believe no further cooling can
occur. Using a powerful electromagnet, the researchers suspended the cloud
in an ultra-high vacuum chamber, until it formed a frigid, swamp-like goop
of atoms



Dennis



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