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I've been thinking, imagine you have a room, a light bulb at the top and you hold your hand up off the ground. Of course, there will be a shadow under your hand and no light will have access to that area, since the hand is blocking the light (the electromagnetic waves). So why am I not seeing clear black, and I'm seeing the dark version of the floor color, the shadow is kind of black transparent, gray... Why am I not seeing complelety black, or maybe the light reflects to the walls of the room and still happens that some very little light access the shadow area? When I move my hand closer to the ground, the shadow goes darker and darker.

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Light is reflecting from the walls to illuminate the area under your hand.

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  • $\begingroup$ Hmh, I was also thinking that probably my hand lets a little bit of light, so that's why the shadow is not completely black? So if I had no walls (Just light bulb, hand and floor), the shadow will be completely black, or at least darker? $\endgroup$ – joro.jur Jan 22 '18 at 20:38
  • $\begingroup$ Yeah, there's also some light reflecting off the walls and then reflecting off your hand to the floor. But every time you add a reflection off of something that's not a mirror, you lose a lot of intensity, so most of the light hitting the area under your hand is coming from the walls, and not your hand itself. $\endgroup$ – probably_someone Jan 22 '18 at 20:40
  • $\begingroup$ What free online resources would you recommend me to learn physics on my own? (I use Khan Academy and relatives, is that enough?) I am thinking about getting into physics seriously and it's the most interesting thing in the world. I am studying physics in school (9th grade, 15 y/o) but I want to push myself more. $\endgroup$ – joro.jur Jan 22 '18 at 20:48
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An alternative way to look at the situation. Imagine you were in space, with the sun shining on you as you traveled along in your spaceship, with its lights off. If you passed behind a planet or a moon, in its shadow, you would experience darkness. How much darkness?

a. If your spaceship was between earth's moon and the earth, you would be in the shadow of the moon, but the side of the earth you were looking at would of course be in daylight. Then, your darkness would only be partial as the earth would light up not only the dark side of the moon, but your spaceship.

b. If, as you passed behind the planet or moon, in its shadow, there were nothing nearby in the direction of the dark side except your spacecraft, then it would be virtually absolutely dark.

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