You might see this trick going viral recently. How does the mirror know what’s behind the paper?? What’s happening?! We can explain!
[Video Description: Barbara, a red-haired white woman wearing a black long-sleeve shirt, stands in the foreground in front of a hallway. In the background, there are various plants and plant art by the white walls.
03:33:14: A diagram on the right shows a piece of paper (white squiggly line) on a mirror (blue line) with an object (orange ball) placed on the paper. Pink dashed lines show the path of light (from the object to the mirror to the eye). Gray dashed line shows the mirrored image as a result of light reflection.
03:22:17: A full-screen image: This video is sponsored by RIT/NTID Regional STEM Center. Below is a logo of RIT on the left with National Technical Institute for the Deaf on the right.]
Transcript: You might see this trick going viral recently. How does the mirror know what’s behind the paper?? What’s happening?! I can explain! Short answer: angle of incidence = angle of reflection. The light is all around, hitting everything in its path from all angles. So, when you put an object behind a piece of paper, the light from the surroundings will scatter off the object in different directions. Some of the light bouncing off the object will reach the mirror just beyond the edge of the paper at a certain angle (also known as the angle of incidence). Since the mirror is a flat, smooth surface, it reflects light at the same angle at which it arrived (also known as the angle of reflection). For example, if the light hits the mirror at 40 degrees, it bounces off at 40 degrees as well. If I stand at the correct angle — in the path of the reflected light, my eyes will receive the light and, thus, see the object behind the paper. What would happen if I moved to the side, changing my vantage point? I now cannot see the object behind the paper. This is because I stepped out of the reflected light’s path, making the object invisible to me. If we can see the object on some surfaces like mirrors, glasses, metals, etc, why cannot we see the object on other surfaces like a piece of paper? If the light rays encounter a rough surface like a piece of paper, they will be scattered in all directions, with each reflected ray not parallel to the next. Therefore, the object’s light that reaches our eyes is not coherent, and we cannot see an image of it. If the surface is smooth, every ray of light will be reflected perfectly from the surface, thus allowing us to see a clear image. Pretty neat party trick, right?!
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