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.
If you mean why do projectors not work on far away things like lasers and need a wall to be clear? It is because the light coming from a laser is very focused and uniform, which helps it keep together and not get scattered over long distances.
Light coming from a projector on the other hand is spread out wide by the projector to show a whole imagine on the wall/screen it is designed to be used with. Removing that wall/screen the projector light continues traveling and spreading out and getting scattered by dust or the air to the point where when it does hit something you cant make it out as the image.
* You can only see light that enters your eye.
* Everything you see on a projector screen has bounced off the screen and into your eye.
* When you see a laser “in mid air” you’re really seeing the photons of the laser bounce off of something in the air (like dust or haze that has been intentionally pumped into the air).
* The major reason you see this with lasers and not with projectors is:
* Lasers usually are part of a light show, so the whole purpose is to see them in the air.
* For that reason “theatrical haze” is pumped into the air to give the lasers something to bounce of off.
* Also lasers are very focused so there is a lot of photons squeezed into a tiny beam.
* Projectors, on the other hand, are designed to show images clearly on a large screen.
* So they are pointed so that the photos spread out to cover the size of the screen.
* Also in most places where you’d see a projection screen, you wouldn’t use theatrical haze (unless it’s a stage show).
A laser emits far more light in a far more concentrated area than a projector. The few flecks of dust that are in the air in the lasers path will deflect some of that laser light, which makes it into your eye, giving you the idea of seeing the lasers path.
With a projector, the light is so much weaker that any small part bouncing off the dust is so dim that you don’t really see anything.
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