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



Prometheus wrote:
> 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.

At last someone has mentioned the word "refraction". We learned it at
school, 65 years ago! What DO they teach kids nowadays?

Dennis.



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