Re: Why is the sky red?

From: Terry B (bohlsen_at_nospam.northnet.com.au)
Date: 11/30/04 

Date: Tue, 30 Nov 2004 21:19:30 +1100

"Prai Jei" <pvstownsend@zyx-abc.fsnet.co.uk> wrote in message
news:coao0g$qgg$1@newsg4.svr.pol.co.uk...
> Jimmy (or somebody else of the same name) wrote thusly in message
> <0HFld.121844$dP1.419958@newsc.telia.net>:
>
>> Hi! Some days when the sky goes up or down the sky is very very red so
>> the
>> air around us looks red to. Why is this happening?
>>
>> Guess there must be something special with the air those days that
>> changes
>> the frequency of sunlight that we observe.
>>
>> Thanks
>
> The blue of the sky is due to reflected (scattered) light being mostly
> blue
> while the red of a rising or setting sun is due to transmitted light which
> is correspondingly richer in red.
>
> For the same reason blue-eyed people (myself included) have their eyes
> appearing red in flash photographs. There is no blue or red pigment
> involved. The "normal" blue arises from scattering of light by the cloudy
> but *unpigmented* iris. The red-eye effect arises from the flashlight
> passing through the iris (so losing its blue by scattering) and being
> reflected back by structures in the eye behind it.
> --
> Paul Townsend
> Pair them off into threes
>
> Interchange the alphabetic letter groups to reply

Sorry but the red eye effect has nothing to do with the colour of the iris.
Normally the pupil looks black because almost all of the light that enters
the eye is internally reflected. If the source of light is nearly on the
same axis as your eyes then the light is reflected back by the retina and
you see a red colour from the blood in the retina. This is how an
ophthalmoscope allows you to see the retina. A mostly parallel light source
is beamed onto a mirror just below the optical axis and reflected to be
parallel with the vision of the observer. i.e. you look just over the top of
the source of light. This light is aimed into the pupil and is reflected
back by the retina. The image is magnified and focused by a lens. If the
pupil is small due to bright light it is harder to visualise the retina. If
it is dilated due to darkness (or eye drops) then it is easier.
The same principle happens with flash photos. The pupil is dilated due to
the dim light (hence the need for flash) allowing the flash to reflect back
to the camera lens. The effect is prevented by either using a preflash to
make the pupil constrict or by moving the flash further away from the
optical axis of the camera- a difficult problem with todays compact cameras
but easy with a separate flash on a SLR.

Clear skies

Terry B
Moree
Australia

Relevant Pages

  • Re: Accommodation in Aphakial Eyes -- Bates has not been disproven on this.
    ... noticeable to the naked eye. ... about the normal eye is how much the pupil is constantly adjusting for ... I think it is true that the amount of sun energy entering a healthy ... And importantly the retina has the ability to adjust its light ...
    (sci.med.vision)
  • Re: Dark adaptation on exit pupil size
    ... dark adaptation is primarily a chemical process within the eye. ... > fraction of the total improved light sensitivity of the dark adapted eye. ... > not aware of research showing a correlation between pupil size and naked eye ... eye is in the retina, ...
    (sci.astro.amateur)
  • Re: Dark adaptation on exit pupil size
    ... >eye is in the retina, pupil size is important too. ... >not change (eye already full dark adapted) this could be significant. ... Here's a site that allows you to model naked eye limiting magnitude ...
    (sci.astro.amateur)
  • Re: blue eyes
    ... In short, no, the color of the eye does not affect the colors ... The black pupil of the eye is basically the hole ... with the amount or wavelengths of light that reach the retina. ... The purpose of the iris is to control the diameter of the pupil, ...
    (sci.med)
  • Dark adaption and pupil size - an experiment (longish)
    ... when I was about six years old looking though a neighbors reflector ... The 11x80 bincos were very impressive from a dark ... I'd do an interesting experiment to see if increasing my pupil size ... often used in eye drop form by eye doctors to dilate patients ...
    (sci.astro.amateur)