1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light and sight and time?

  1. Sep 23, 2013 #1


    User Avatar

    Can someone please explain how vision works? When we see everyday objects how does it work? Is light reflected off the objects and then back into our eyeballs?!? I always heard when you look up to the stars it's like looking into the past. Even our sun, when we see the sun we're seeing the sun as it was 8 minutes ago. Can someone explain this further? And When we see everyday objects are we seeing them as they were billionths of a second into the past? And if so, is this due to the brains processing speed, or what causes that? And if looking great distances away is like a time machine...say there was a gigantic mirror in space that is 1 light year away and we had a really good telescope, could we look at that mirror and see earth 2 years into the past? This is so very interesting to me, any response would be greatly appreciated. :)
  2. jcsd
  3. Sep 23, 2013 #2
    Yes, light reflects off of objects and hits your eyes. Your eyes detect light. But also, objects emit light and your eyes can detect that too. Light takes time to travel over distances, it does not travel instantaneously. The further the light has to travel, the longer it will take.
  4. Sep 23, 2013 #3


    User Avatar
    Science Advisor

    Yes. The light from the object reflects. Part of that light strikes in the area of your pupil. This is the hold in the center of the iris through which you see. Your pupils adjust to brightness, getting narrower in bright light and wider in dim light. This helps equalize the illumination levels on the retina, preventing damage in bright light while helping you see better in low light.

    The light then passes through a lens in your eye. This is immediately behind the pupil. It acts to take diverging rays of light coming from a point in your field of view and cause those rays to converge on a point on your retina. If there is an object in your field of view that is reflecting light toward your eye, the lens will focus that light on a point in your retina.

    Light sensors on your retina detect this illumination and send a signal toward the brain throguh the optic nerve.

    There is a problem -- diffraction. The best angular separation that you can resolve is a function of the width of the telescope and the wavelength of light you are using. For visible light with a wavelength of 580 nanometers and a telescope with an aperture the size of the earth, your resolving power at 2 light years is only good enough to resolve an object about 100 times the size of the earth (assuming I haven't slipped a few digits somewhere).

    Rather than standing up a huge mirror 2 light years out and putting together a huge telescope to look into it, it's a whole lot more cost effective to put a camera on the street corner and save the tape for a couple of years.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Light and sight and time?
  1. Light & Time (Replies: 8)

  2. Light and time (Replies: 6)

  3. Time and Light (Replies: 2)

  4. Light & Sight (Replies: 58)