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



In article <1145858451.759527.73370@u72g2000cwu.googlegroups.com>,
cjcampbell writes
>
>Dave Hillstrom wrote:
>> You can't use regular lenses underwater, that's why you always see
>> special cameras on ocean documentaries and such. People think it's
>> because they need to be waterproof but the real reason is the speed of
>> light underwater is different than in air and it makes the light bend
>> at different angles through the lenses.
>> Einstein proved it.
>
>So, I suppose that is also the reason you need different lenses to take
>telephoto pictures than the lenses you would use for wide angle. Light
>from different distances travels at different speeds? No doubt this is
>due to the fact that light traveling from farther away has to go
>through more haze, so it passes through more water on the way to the
>camera. It would explain depth of field, too. A lens set at a
>particular focal length can only bend light traveling at a particular
>speed at the correct angle. Light that is going too fast or too slow
>would, of course, bend at the wrong angle and thus the foreground and
>background are out of focus. That being the case, you would also need
>special lenses in space, where light does not travel through any water
>at all.
>
>Or maybe it is just wrong to encourage this nut.

No, it is you who is wrong. Light DOES travel at a lower speed through
water than air The refraction at the water/glass interface of the front
element when immersed in water is very different to the air/glass
interface when in air. The effect of water vapour in air is much, much
less; although droplets will cause random refractions and make objects
blurred.
--
Ian G8ILZ

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