MadSci Network: Chemistry

Re: What is the difference between polarized sun glasses and those advertizing

Date: Mon Jun 12 09:41:47 2000
Posted By: Tom Stickel, Grad student, Optometry, Indiana University School of Optometry
Area of science: Chemistry
ID: 960219874.Ch

Dear John,
  All sunglasses should have UV protection, but not all sunglasses need to 
be polarized.  UV is short for ultraviolet, which refers to a kind high 
energy  electromagnetic radiation that we can't quite see.  The wavelength 
ultraviolet radiation is just a little shorter than the color violet.  
Anyway, because shorter wavelength electromagnetic radiation has more 
energy than longer wavelength, "visible light," it can damage your eyes.  
This is why you should always buy sunglasses with 100% UV protection.  UVA 
and UVB refer to different wavelengths of ultraviolet radiation.  UVA 
(ultraviolet A) is a little bit shorter than violet light, and UVB is even 
shorter than UVA.  This means UVB has more energy than UVA and so can cause 
more damage to your eyes. The sun also emits UVC, which has even more 
energy, but your cornea pretty much blocks all of this by itself without 
sustaining any damage.
  Sooo, to cut to the chase, if you're going to be outside for a while, 
wear sunglasses that block 100% of the harmful UVA and UVB rays from the 
  Polarized sunglasses are a different treatment that's added to lenses to 
help block out glare off of objects like highways (important for driving) 
and water (important for you as you stare for hours waiting for one little 
fish). The polarization of light is a tricky subject to explain, so here's 
a good website that's a little technical:
  Basically, light travels in waves.  But all the waves aren't parallel to 
each other.  Instead, light vibrates in planes that are spread 360 degrees 
around the clock.  When light hits a reflector like water, what comes off 
is pretty much polarized so it's vibrating parallel to the ground.  
Polarized sunglasses block the concentrated waves that are reflected 
parallel, but they let the perpendicular rays pass through.  These 
reflected waves are responsible for glare, so if you block those out,the 
glare is gone.  But since you are still seeing waves aligned in the other 
directions, you can still actually see.
  But the important part is that it means polarized lenses will let you see 
into the water on sunny days.
  If it's not sunny out, then polarized lenses don't do you any good. If 
there is no glare, there's no point trying to block it out.
  As for what color of lens is best, this is a personal issue.  I like dark 
brown lenses, but most people usually go with grey. This is because most 
sunglass manufacturers sell gray.  I think brown lenses give you more 
contrast than gray and make the world look a little "happier."  Test drive 
both types before you buy.
  For those grey, low contrast type of days, try out yellow lenses to 
sharpen up details.  Make sure they still have ultraviolet protection, 
though, since UV radiation goes through clouds.  I don't think the yellow 
will get you to be able to see into the water any better, but it will 
brighten things up. 
  Hope that helps. Good luck fishing!


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