I know lasers are able to retrieve data (HDD, laser distance meters, optic telescopes etc.) but how does one do it? Let’s take distance meter as an example, let’s say I want to measure how far away I am from a wall. No wall is 100% smooth, so when I point a laser beam into it it should bounce off in some unkown place due to rigidness of the wall. I mean as far as I am concerned laser beam should bounce off of the wall and go back to the detector, and it does its magic and it somehow knows how long did the light traveled. But rigidness of the wall should make it bounce somwhere (angle of incidence equal to the angle of reflection).
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I’m not sure ‘rigidness’ is what you want here. Something is rigid if it does not bend, but the bendiness of a wall doesn’t really affect whether or not you can measure distance with a laser. If your laser is strong enough to bend even a very weak wall, you have bigger problems than distance.
Either way: you’re right that it would be hard to measure distance with a laser if the wall perfectly reflected the laser off in another direction – but that’s only going to happen with a mirror. A laser beam is like a cylinder just packed full of light, and when it hits a normal wall some of that light is going to bounce one way, some of it’s going to bounce another, and at least some is going to bounce back to where it came from.
(This, by the by, is why you can see the laser dot on the wall – some of that light bounced back to your own eyes! It’s also why looking at the dot is safe but looking directly at the laser is *not* safe – if you look directly at the laser, then *all* that light goes into your eye and can hurt you.)
The way you measure distance with a laser is to not send out a long-lasting beam, but instead pulse a little bit of laser at the wall. Then, you measure how long it takes for the laser pulse to bounce off and (at least in part!) come back to you. You need a really good sensor and a really good clock, since it’s going to happen *very very fast*, but we know how fast light moves in normal air and if we know how long it took, it’s pretty simple math to figure out how far we are from the wall.
For your other examples: HDD (a hard drive) doesn’t use a laser, but rather magnets. An *optical drive* (like a CD, DVD, or Blu-ray) does use lasers, though – the surface of the disk has a bunch of very very tiny pits burned into it, and if we focus a laser onto the disk while it spins we can watch how the laser’s reflection changes as it hits all the little pits. That gives us a pattern we can turn back into data.
> I mean as far as I am concerned laser beam should bounce off of the wall and go back to the detector, and it does its magic and it somehow knows how long did the light traveled.
Just addressing this; it’s not really magic. All you have to do is start a timer when the laser pulse goes out and then stop it when you detect some of the reflected light returning. You can then just take that amount of time, multiply it by the speed of light, and you get your distance.
The only thing that would actually completely reflect the light off into a different direction is going to be a mirror, and even those aren’t perfect.
So the tl;dr is that basically, no material is going to do what you think is happening. At least *some* of the light will get bounced back directly to where it came from so it hits the detector and thats how we measure distances.
As for how they do that, we send a really quick pulse of light and then measure how long it took for the light to be seen by the detector.
ELI5: How do we ensure that IR beams in IR proximity sensors reflect directly back to the sensor?
byu/jso__ inexplainlikeimfive
If the surface isn’t a mirror, it scatters the reflection. This diffuse reflection is enough for the detector.
For distance measurement that doesn’t require crazy accuracy, the laser beam will turn off briefly for the measurement. A small camera will capture the time it takes for the laser to reappear. Knowing the speed of light, and a bright red laser is easy to spot, it’s not hard.
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