[Tfug] Another OT Optics Question

[Paul Scott]

Bexley Hall wrote:
> Hi, Jeremy,
>
> --- On Sun, 8/3/08, Jeremy D Rogers <jdrogers at optics.arizona.edu> wrote:
>
>   
>>>> 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 think Hu's right, but just to be clear, if the
>> 'angular' size is
>> always ~40deg, then it always appears the same size.
>>     
>
> Sorry, I don't understand this comment.  A rainbow
> from my garden hose "appears" 5 or ten feet in diameter.
> It *never* appears as large as one that I see in the distance
>   
It might be similar to why we think that the moon looks larger when it's
close to the horizon but it's only because it is being compared to the
objects near it. 

Paul Scott