Re: How does eye damage occur when looking at sun during total solar eclipse

Trudy,
  Although pupil constriction does do a reasonable job of controlling light 
levels that reach your eye, it really won't do much to either harm or 
protect your eyes if you look at the sun in any situation. 
  The important damage caused by the sun is damage to your macula.  This 
area of the retina is responsible for your central vision, which is also 
your area of highest visual acuity.  If this small area is damaged, then 
your vision can drop from 20/20 to 20/400 even though only a few degrees of 
the total visual field has been compromised.  
  Since the macula is pretty much straight behind the pupil, it doesn't 
really matter whether the pupil is dilated or constricted.  Either way, 
light gets focused on the macula, and this is what causes the damage to the 
eye.  It's like lighting paper on fire with a magnifying glass; a little 
light goes a long way when it's focused correctly.  Likewise, your eye is 
built to focus light rays precisely on the macula for optimum vision.  But 
when you look at the sun, or even the edges of the sun around an eclipse, 
you are focusing a whole lot of photons on your macula.  The other big 
problem is that the sun emits even more infrared radiation than visible 
light.  Infrared radiation is hot stuff, so when focused it causes damage 
just like the visible light.  Interestingly, ultraviolet radiation doesn't 
cause much damage to the retina or macula during a solar burn, even though 
it has more energy per photon than either visible light or infrared.
  So the the key is that even looking at the edges of the sun, such as 
during a solar eclipse, is a bad idea.  A little direct solar radiation can 
cause a lot of damage because of the way it's focused on your eye.  As an 
aside, this is why looking at a laser pointer is a bad idea too.  Lasers 
don't contain that much light, but they are highly concentrated over a 
small area.  When someone looks right at the light and gets those 
concentrated rays in the macula, problems happen.  
  There you go, and I hope this helps out a little.  Write back if you have 
any questions!

Tom

[Moderator's note:
During an eclipse, it is safe to look at the Sun with the naked eye during 
totality, when only the corona is visible. In fact, you won't see anything if 
you look at totality through mylar filters, for example.

There's a good article about eye safety, by Ralph Chou, at 
http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety2.html

- Jim O'Donnell]