This is one of my favorite demos. I like it because anyone can do it at home and people usually find the results surprising. Here is the situation:
How much of yourself could you see in a small mirror? What if you move farther away from the mirror, can you see more of yourself? If you want to do the demo yourself, now would be a good time. All you need is a small mirror (I used a platter from a 10 GB hard drive – they make awesome mirrors). It will help out a lot if you can mount the mirror on a surface that is very near to vertical.
Here is my version of this demo (in case you can’t find a mirror).
To answer this, I will first answer a bonus question. How big of a mirror do you need to see your whole face. (I am going with the face because that is similar to the above video demo) The only things you really need to know are:
- In order to see the top of your head, light has to reflect off your head and to your eye
- Light reflects off the mirror at the same angle it hits it
I am not going to show where the light comes from, just to make it look less-crazy. But here is a diagram for a person looking at a small mirror.
I didn’t really give away the question to “how big of a mirror do you need?”. Anyway, what if you move farther away from the mirror? Here is another diagram.
The person can still see all of his/her face with that size mirror. It doesn’t matter how far away the view is from the mirror.