[Tfug] Another OT Optics Question

[Hubert M Bath]

More light (pun intended) is shed on the rainbow problem by the 
Wikipedia article at  http://tinyurl.com/6k29y.  Apparently my 
understanding of what is going on is about 400 years behind that of 
Descartes, who had the thing pretty well doped out in 1637.

Regards,  Hu Bath

*************

Hubert M Bath wrote:
> Bexley Hall wrote:
>> OK, zero-th order understanding of how rainbows manifest
>> themselves...  :>
>>
>> Inceident ray comes back ~40 degrees refracted (and reflected).
>> So, can I think of rainbow as a "conic section" in a cone with
>> a 42 degree apex?
> 
> The 42 degrees applies to red light, and 40 degrees to violet, the 
> inside of the "primary" rainbow. These angles apply no matter the height 
> of the sun.  Only if the sun is at the horizon, and sun's rays therefore 
> parallel to the earth's surface does the rainbow appear as a semicircle 
> --with its center at the horizon.  If the sun is higher, the rainbow 
> center is below the horizon and the rainbow less than a full semicircle. 
>     If the sun is not so low, the rainbow moves down, so that its center 
> is below the horizon, rather than at it.
>> And, the apparent "diameter" of the rainbow is a function
>> of how far the water is from the observer?  E.g., a rainbow
>> thrown from the mist of a gargen hose would appear to have a much
>> smaller diameter than one from water vabor many miles distant.
> 
> I think that the *angular* diameter remains the same, tho, obviously, 
> the actual refraction-reflection-refraction is occurring much closer 
> than if the drops were from a distant rainstorm.  And, because the sun 
> is presumably 'way above the horizon, the the arc of the spraybow would 
> be less than a semicircle.  In fact, if the sun is more than 42 degrees 
> above the horizon, a rainbow is impossible (because its "center" is more 
> than 42 degrees below the horizon.)
>> (I'm not looking for "6 decimal places", here... just a crude
>> understanding of the geometry involved  :>  )
>>
>> Thx!
>> --don
>>
>>
> Regds,  Hu Bath
> 
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