eli5 On projectors, why does the screen show only on the wall, rather than being carried out like a laser?
In: Technology
Do you mean to ask why you don’t see the image hang in mid-air, in the way [laser beams at a show do](https://lightdiction.com/photos/Monaco/IMGP2548.JPG)?
That’s because to be visible, light needs something to bounce off, to make it into your eyes. Normally that’s the wall, because air normally doesn’t have much stuff floating in it. To do something like a laser show, the air needs to have smoke or dust in it. A very bright laser can be visible in a normal room, because there’s always small particles of dust flying around, but that takes a lot of power, and makes for a very dangerous laser. In the same way a projector bright enough to produce an image in mid-air would be blinding.
There exist experimental devices that can produce a [screen in mid-air by blowing smoke](https://www.youtube.com/watch?v=rw9T-n_Ay5k#t=10m30s) in a sheet.
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