Dot Physics

A demo for the color black

What do you see when you are in a completely dark room with no lights?

That is a great question to ask. It can bring out some interesting ideas. Anyway, here is an easy demo to show the color black. The basic idea is to build a box that has a small opening. Here is what it looks like from the outside:

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As you can see, just a basic box. I have a door on the top, and I put a paper towel tube for a window. To make it look pretty, I covered it with black paper (so you couldn’t tell where I stole the box from).

To demo this to students, I first go around and let everyone look inside. I ask “what color is the inside?” Most people will say black, but some will say “I can’t see anything”. If you press them, they will agree it is black (stress – what color is it?). This is what they see:

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The next step is to open the door on the top and let them look inside. Or, if you like, you can ask what color the inside of the box actually is. The real reason I colored the outside of the box black is so that they would perhaps assume the inside is also black. With the door open, this is what a student would see inside:

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In case you are curious, that is a picture of me with a burger king crown on (long story). You could put anything in the box that you think is amusing. Hopefully, students ‘see’ that if no light is reflecting off my image, then you won’t see it (which is what we call black). This box is actually a type of black body. Although light does go into the tube, it gets absorbed a little each time it reflects off an inner wall. By the time any light would reflect back out, it has been mostly absorbed.

Here is the next-level question:

Suppose I mount a mirror on the back wall of the inside of the box. If you keep the top lid closed and look in, what would you see?

Here is a picture if you can’t handle not having the answer. As a note, I put a platter from a hard drive back there and had trouble lining it up. But you get the idea.

Comments

  1. #1 Kyle Allain
    July 8, 2009

    Rhett – you come up with the coolest blog posts! Great stuff – really stimulates the synapses in the noggin’ to ponder such things!

  2. #2 Frank Noschese
    July 8, 2009

    Awesome! I was initially confused by the picture with the mirror, but I get it now after a bit of thinking. Thanks for sharing!

  3. #3 Uncle Al
    July 9, 2009

    Ligtly bolt together a registered stack of degreased double-edged razor blades. Be careful not to ding the sharp edges. The two sharp sides are eye-sucking black despite (because of!) the polished mirror blade edges. (Store with silica gel desiccant – they rust.)