Re: [PSUBS-MAILIST] One more reason to use acrylic windows.

["Sean T. Stevenson"<sts@telus.net>]

Refractive index is a material property.  It may be possible to manufacture
custom polymers to create plastics with a desired refractive index, but this
property is constant in homogenous solids.

The optical behaviour follows "Snell's Law", which is sin theta1/sin theta2=n1/n2,
where the theta values are the angles of incidence of light with respect to
the surface normal, and the n values are the refractive indexes.  From this,
you can see that light passing from one material to another does not change
angle at all if it enters directly perpendicular to the interface.  The angular
difference (refraction) becomes greater as the incidence angle increases.

For seawater, with an n value of 1.37, any light hitting the surface at an
angle greater than 46.88 degrees will reflect off instead of passing through.
 This is why as a diver, provided there is sufficient visibility, when you
look up the surface light is a circle of light centered above you.

-Sean



>I remember hearing somewhere that Graham Hawks Deep Flight canopy was
>manufactured to couple with seawater so well that it seemed to disappear
>when the sub was placed in the water.  My question is, does the conical
>shape and thickness of the dome or viewport effect the refractive index?
>And if so, is there a specific radius or cone that is "best" for underwater

>viewing?
>
>James
>
>----- Original Message -----
>From: "Sean T. Stevenson" <sts@telus.net>
>To: <personal_submersibles@psubs.org>
>Sent: Thursday, May 30, 2002 3:45 PM
>Subject: Re: [PSUBS-MAILIST] One more reason to use acrylic windows.
>
>
>> The "optical erasing" that you refer to is the result of the refractive

>index
>> of acrylic being closer to the surrounding water than other materials.
>>
>> For reference, some approximate values are:
>>
>> vacuum          1.0
>> fresh water     1.33
>> sea water       1.37
>> acrylic         1.49
>> glass           1.52
>> polycarbonate   1.58
>>
>> -Sean
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