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



In article <1hei6t0.p8ekho18s5fd9N%here@now.au>, obt
writes
>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...

You evidently were not paying attention at school, it was Maxwell who
showed that the speed of light in a vacuum must be constant and gave the
value through the his equations, Einstein developed his Theory of
Relativity using this constancy. In simple terms the constancy of the
speed of light in a vacuum is one of the two postulates that gave rise
to the theory of relativity, and not the other way around.

Read this first, then progress through the other articles.
http://www.physicsclassroom.com/Class/refrn/u14l1d.html
You will find at the bottom of the page an equation linking refractive
index (n) and the velocity of light in a material (v_material) and a
table giving the refractive index for a number of materials. If you
transpose the equation you can obtain the velocity in the metrical, for
water it is @2.5e8 m/s and for Gallium phosphide 0.86e8 m/s.

General notes
http://www.thebigview.com/spacetime/index.html

Maxwell
http://scienceworld.wolfram.com/physics/MaxwellEquations.html
You might try
http://www.sfu.ca/physics/associate/emeriti/cochran/MAX.pdf

Einstein's 'On the Electrodynamics of Moving Bodies'
http://www.fourmilab.ch/etexts/einstein/specrel/specrel.pdf

It would seem that yet again we have proof, if it were needed, that "A
little learning is a dangerous thing" [Pope, An Essay on Criticism
(1709)].
--
Ian G8ILZ

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

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