Let’s say you are in a rectangular shaped room and on the far end is a single standard 60 watt lightbulb and at the other end you are standing observing the lightbulb. The room is completely black including the walls, ceiling and floor with no glossy sheen on the surface; so absolutely no reflective surface.

When looking at the lightbulb on the far end of the room you can see in your line of perspective the bulb is the size of your thumb due to the distance.

So my question is:

If you are standing on the opposite end of the room from the lightbulb and you cover it up in your line of sight with your thumb will the room go dark. Let’s assume no light goes through your thumb and that you are wearing a full body suit that is black and not reflective. Will the room go dark from your perspective?


1 Answer 1


Even Vantablack reflects about 0.03% of light, but let's assume that you have something better, perfect. Also assume you are a cyclops (or have two thumbs) and your thumb is covered by the bodysuit (which in addition to being 100% black is also 100% opaque).

Also assume that you are in a vacuum so there is no scattering off of air or dust in the air.

Ignore the fact that the walls are at a finite temperature, and so will produce a small amount of blackbody radiation at all wavelengths. (This is actually not important because your eyeball is hotter than most rooms, so its internal blackbody light will dominate.)

What you will see is light diffracting around your thumb. It will look like your black thumb is surrounded by a thin halo of light that comes from the lightbulb and curves around your thumb into your eye. If your thumb is large enough (still small compared with your whole field of view) the diffracted light will be negligible.

In that case, everything will be dark. You will not be able to tell by looking whether you are in a small room or an infinite starless universe by looking at the walls. You will not be able to tell whether the lightbulb is on or off.

See the video here to see what black looks like.

  • $\begingroup$ Let’s change things a little, what if your thumb in your line of sight was perceived as twice as big as the lightbulb. Would light still refract around it in a halo or will the room appear to be absolute darkness? $\endgroup$ Nov 27, 2019 at 18:50
  • $\begingroup$ "Your eyeball is hotter than most rooms" is somehow one of the more disturbing sentences I've read today... $\endgroup$ Nov 28, 2019 at 16:16
  • $\begingroup$ I could have phrased it: 'The room is so dark that it actually sucks light out of your eyes' if I wanted to be disturbing. $\endgroup$
    – DMPalmer
    Nov 28, 2019 at 19:29

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