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!

Black, reflected and emitted

  1. Oct 30, 2013 #1
    Hi all. I have a couple questions regarding the “color” black.

    First question, regarding reflected light:

    Would a perfectly black object with no reflectiveness to its surface absorb 100% of the light that hits it? If so, would it activate no rods/cones on the observer’s retina whatsoever, and essentially be a blank spot in their vision that appears pure black? Is such a thing even possible (outside of a black hole)?

    Second question, regarding emitted light:

    I know that some TVs and other types of screens are capable of producing darker blacks than others. How can something that emits light emit black at all when it is actually the absence of light? Wouldn’t it just not activate its LEDs (if it’s an LED display) for the areas of the screen it wants to be darkest?

  2. jcsd
  3. Oct 30, 2013 #2


    User Avatar

    Staff: Mentor

    You're talking about what's known as a perfect black body. They don't actually exist, except possibly a black hole, but you are correct. It would absorb all radiation falling on it and you would not be able to see it at all except as a dark spot.

    I don't know the answer to your 2nd question.
  4. Oct 30, 2013 #3
    Thank you!

    As for question #2, it looks like it depends on the type of display. A plasma TV will actually turn off pixels to display black, but an LCD will block the light being emitted by a diode. I'm finding this information here: http://www.rtings.com/info/lcd-vs-led-vs-plasma/how-they-work
  5. Oct 30, 2013 #4
    Question #2 got my interest. I've seen CRT screens that look dark grey when turned off. But when I turn the computer on, the screen actually turns black. How is that possible? Optical illusion? Or with the grey, and I getting light from inside the monitor, but that light is blocked when the screen turns on?

    As for the first question, we have not succeeded at making anything 0% reflective or 100% reflective. We've gotten down to 0.3% reflective, but you would still see some faint reflection on the surface, making it not true black. They can also put baffles on the surface so that light hitting the part you are looking at is reflected away from your eyes, so you don't see it. Or with a reflective surface, they can make an image appear right in front of you so that if your eyes are at just the right location, you will think you are looking at something solid.
  6. Oct 30, 2013 #5
    Thanks for the reply, Stargazer!

    I don't see how there could be light coming from inside the monitor when it's off, but maybe when it's on, the pixels of the screen that block the light are actually a truer black than when they are completely off. I don't know if this is possible.

    Would this be a hologram?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook