B Light beams and their reflection

[elou]

TL;DR Summary

Pointing a laser beam obliquely at a mirror, or even next to the mirror, missing the mirror altogether, the mirror will show the glowing head of the laser in the darkness, just like it would show everything facing the mirror in a lit space.
Smoke spray makes the beam visible, but nothing can be made visible that links the laser to its reflection on the mirror.

I must add, that, when used in a lit space, or in daylight, spraying before the mirror does not make any light rays visible. Is it simply a matter of light scattering?
Also, I am not sure whether this question belongs here or in classical physics.

 

[hutchphd]

What does "the mirror will show the glowing head of the laser in the darkness, just like it would show everything facing the mirror in a lit space" mean ? It is unclear.
Is this an "ordinary" glass mirror aluminized on the back surface? Could you be seeing a reflection from the front surface? Perhaps multiple internal reflections?

 

[Dale]

Are you talking about virtual images? It is hard to tell what you are talking about

 

[elou]

No, simple reflections. Standing in front of the mirror and shining a laser beam. You can always make the beam visible with smoke. But not the path from the laser to the reflection of the mirror. When the path of the beam is made visible, it appears of course also on the mirror.
edit: the reflection of the beam also stands alone with nothing linking it to the laser.

 

[Ibix]

Do you mean, even if you don't point a laser at a mirror, you can see light from the laser device in the mirror? Maybe, if the muzzle of the laser emits light apart from the beam. You often get glow from, e.g., a HeNe laser medium

 

[elou]

Do you mean, even if you don't point a laser at a mirror, you can see light from the laser device in the mirror? Maybe, if the muzzle of the laser emits light apart from the beam. You often get glow from, e.g., a HeNe laser medium

Possible. But I still cannot explain how I can see the laser beam in the smoke, and its reflection on the mirror, even though there is nothing visible linking the beam made visible, and its reflection on the mirror.

edit: if you point the laser at the mirror, you see of course the oint of impact, but also the reflection of the glowing element. But even if you make the path to the mirror visible with smoke, still nothing connects the glowing head with its reflection.

 

[Ibix]

But I still cannot explain how I can see the laser beam in the smoke,

Scattering off the smoke particles.

and its reflection on the mirror,

Scattering off the smoke particles reflected in the mirror.

 

[elou]

Scattering off the smoke particles.

Scattering off the smoke particles reflected in the mirror.

That is what I would think also. But the scattered particles have to come together again, and shouldn't smoke then make them visible?

 

[elou]

What does "the mirror will show the glowing head of the laser in the darkness, just like it would show everything facing the mirror in a lit space" mean ? It is unclear.
Is this an "ordinary" glass mirror aluminized on the back surface? Could you be seeing a reflection from the front surface? Perhaps multiple internal reflections?

Normal everyday mirror. In fact, a bathroom mirror. No multiple internal reflections.

 

[Ibix]

But the scattered particles have to come together again, and shouldn't smoke then make them visible?

I have no idea what you are trying to say here.

This is fairly simple. Light comes out of the laser. It bounces off a smoke particle. It travels to your eye. You see it coming from the direction of the smoke particle it bounced off.

 

[Dale]

No, simple reflections. Standing in front of the mirror and shining a laser beam. You can always make the beam visible with smoke. But not the path from the laser to the reflection of the mirror. When the path of the beam is made visible, it appears of course also on the mirror.
edit: the reflection of the beam also stands alone with nothing linking it to the laser.

I think that you are talking about a virtual image.

https://en.m.wikipedia.org/wiki/Virtual_image

“A plane mirror forms a virtual image positioned behind the mirror. Although the rays of light seem to come from behind the mirror, light from the source only exists in front of the mirror.”

there is nothing visible linking the beam made visible, and its reflection on the mirror

The beam made visible literally intersects its reflection in the mirror at the mirror. So in what way is there nothing visible linking them?

 

[elou]

I have no idea what you are trying to say here.

This is fairly simple. Light comes out of the laser. It bounces off a smoke particle. It travels to your eye. You see it coming from the direction of the smoke particle it bounced off.

It would make sense if the smoke did not show exclusively the path of the beam. There is also nothing linking me with the laser that the smoke made visible. The beam, and nothing else.
I have been puzzled by this phenomenon a very long time, asked the magazine Nature since they offer the possibility to ask questions, and I never got a response.

 

[hutchphd]

An "ordinary" mirror has two polished surfaces, one of which is "silvered" Each surface will reflect light. But, like everybody else, I know not what you are trying to say here. Sorry.

 

[Ibix]

It would make sense if the smoke did not show exclusively the path of the beam

Why would the smoke be bright where there isn't anything illuminating it?

 

[elou]

I think that you are talking about a virtual image.

https://en.m.wikipedia.org/wiki/Virtual_image

“A plane mirror forms a virtual image positioned behind the mirror. Although the rays of light seem to come from behind the mirror, light from the source only exists in front of the mirror.”

The beam made visible literally intersects its reflection in the mirror at the mirror. So in what way is there nothing visible linking them?

You can see the path the beam is following, say, somewhere at the right of the mirror. When I spray smoke, I see the beam traversing the smoke, and I see the reflection of the beam traversing the smoke on the mirror. Nut there is nothing linking the visible beam in the smoke, and the reflection on the mirror.
I know it's weird, but that is easily verified. All you need is a regular mirror, a laser pen and a can of smoke spray.

 

[elou]

Why would the smoke be bright where there isn't anything illuminating it?

scattering?

 

[Dale]

Nut there is nothing linking the visible beam in the smoke, and the reflection on the mirror.

They literally intersect at the mirror surface.

I don’t think what you are saying is true. I also don’t think it matters. Why should there be something visible linking an object and its reflection?

 

[elou]

They literally intersect at the mirror surface.

I don’t think what you are saying is true. I also don’t think it matters. Why should there be something visible linking an object and its reflection?

I don't know what you mean. How would otherwise light travel between objects, and between objects and observers?

 

[Dale]

I don't know what you mean. How would otherwise light travel between objects, and between objects and observers?

I think the laser is a distraction. Just look at your own face in the mirror. Do you see anything visible linking your face with its reflection? Then why would you expect anything else to have such a link?

 

[Ibix]

scattering?

But scattering of what? You mean secondary scattering of the already scattered light? Sure, you'll get some of that, but you need fairly dense smoke and bright lasers to see it with the naked eye.

 

[elou]

I think the laser is a distraction. Just look at your own face in the mirror. Do you see anything visible linking your face with its reflection? Then why would you expect anything else to have such a link?

I completely agree with you. And I don't know either how that is possible.

 

[Ibix]

I completely agree with you. And I don't know either how that is possible.

What are you expecting to see? Are you basically asking "why can't we see light while it's in flight?"

 

[hutchphd]

You will need to ask Dracula.......?

 

[Vanadium 50]

This thread seems to be...<ahem>...all smoke and mirrors.

 

[Dale]

I completely agree with you. And I don't know either how that is possible.

Ok, so the laser and the smoke is irrelevant. You expect to see something connecting your face and its reflection, despite never seeing such a thing.

I guess the question is what do you expect to see and why you expect to see it? If the geometry would be confusing with your face, then perhaps talk about your hand and the reflection of your hand.

 

[russ_watters]

Nut there is nothing linking the visible beam in the smoke, and the reflection on the mirror.

I think you're asking why the entire room isn't filled with light? The laser beam I'd several orders of magnitude brighter than it's first reflection (the visible beam) anteater is several orders of magnitude brighter than a second reflection off of other particles in the air. It's just way too dim to see it.

Or: you see the beam because it is directly illuminated by the laser. You don't see the room full of light because it isn't directly illuminated by the laser.

The mirror is irrelevant here. The view of the reflected beam is the same in the mirror or if you are standing right next to the mirror.

 

[Nugatory]

There is also nothing linking me with the laser that the smoke made visible. The beam, and nothing else.

Light is scattered off the smoke particles. Some of this scattered light reaches your eyes - that’s a link. Some of the scattered light reaches the mirror and is reflected into your eyes. That’s the other link.

 

[jbriggs444]

You can see the path the beam is following, say, somewhere at the right of the mirror. When I spray smoke, I see the beam traversing the smoke, and I see the reflection of the beam traversing the smoke on the mirror. Nut there is nothing linking the visible beam in the smoke, and the reflection on the mirror.
I know it's weird, but that is easily verified. All you need is a regular mirror, a laser pen and a can of smoke spray.

Are you expressing amazement that the light travelling from the illuminated smoky beam to the mirror and from there to your eyes does not itself illuminate the smoke particles between beam and mirror and thereby make itself visible?

The light travelling from illuminated smoky beam to the mirror is diffuse. It travels everywhere. The smoke is illuminated by the light going from beam to mirror and from mirror to eyes. But it is illuminated more or less uniformly and much less intensely than from the direct laser light. There is nothing to see but a generalized background glow.

 

[elou]

But scattering of what? You mean secondary scattering of the already scattered light? Sure, you'll get some of that, but you need fairly dense smoke and bright lasers to see it with the naked eye.

 

[elou]

But scattering of what? You mean secondary scattering of the already scattered light? Sure, you'll get some of that, but you need fairly dense smoke and bright lasers to see it with the naked eye.

You will need to ask Dracula.......?

The general consensus is that it is a matter of scattering, but the effect too small to become visible to our eyes, or to the mirror. That is also how far I got.
That created its own problem: if I do the same experiment in outer space, where there is no scattering, would I still see the glow of the laser in the mirror?
Or will the laser, just like Dracula have no mirror reflection?

 

[Ibix]

That created its own problem: if I do the same experiment in outer space, where there is no scattering, would I still see the glow of the laser in the mirror?

There's dust and gas in outer space, so if you have sensitive enough detectors, yes you will see some scattered light from the beam. Otherwise, you will see nothing unless the beam shines directly in your eyes or illuminates an object.

 

[elou]

There's dust and gas in outer space, so if you have sensitive enough detectors, yes you will see some scattered light from the beam. Otherwise, you will see nothing unless the beam shines directly in your eyes or illuminates an object.

So, we won't be able to see the glow of the laser reflected in the mirror, but still be able to see distant stars? That stopped me right in my tracks. Any suggestion?

 

[Ibix]

So, we won't be able to see the glow of the laser reflected in the mirror, but still be able to see distant stars?

The stars emit light in all directions. It's guaranteed that some of it will hit your eyes (unless the back of your head or a planet or something gets in the way). The laser emits light in one direction. It's almost certainly going to miss your eyes unless you go and put your eye in the beam, or some of the light scatters off something.

 

[elou]

The stars emit light in all directions. It's guaranteed that some of it will hit your eyes (unless the back of your head or a planet or something gets in the way). The laser emits light in one direction. It's almost certainly going to miss your eyes unless it scatters off something.

How can the mirror miss?

 

[Ibix]

How can the mirror miss?

How can the mirror miss what?

 

[elou]

How can the mirror miss what?

How can the mirror miss what?

Never mind. I think this discussion has gotten as far as it could.

 

[PeroK]

 

[Ibix]

When light comes from a star it goes in all directions. That's represented by the red arrows going in all directions:

Unless you're somewhere where the light is blocked by something (like behind the mirror) you will intersect a red arrow, and you can see the star. If you happen to be somewhere covered by light coming off the mirror, you can see the star in the mirror.

But if the light source is a laser then the light goes in one direction:

There is light almost nowhere. None of it intersects the mirror (at least, not with the laser pointing where it is). Unless you happen to be in the beam path you intersect no arrows and will see nothing but blackness.

 

[sophiecentaur]

But the scattered particles have to come together again,

How can the mirror miss?

These two quotations make me wonder what is your idea of what is actually going on. I think it's necessary to sum up all the ideas in this thread.

For the purposes of this simple classical physics discussion we can say that light travels in straight lines. If the experiment were in a perfect vacuum, we would see nothing of the laser beam unless it strikes directly into the eye lens. Also, obviously, the beam has to hit the surface of the mirror.

In (dusty) air, the light is scattered away from the laser beam as it hits tiny particles (reducing the energy in the beam as it goes). The particles in the air are not 'scattered'; they are not perturbed by the light beam (ignore very high power light beams - too hard for this discussion). Light travels directly from each scattering point to our eyes (and in all other directions)

We will see the small portion of all that scattered light that comes our way as bright lines, to and from the mirror surface. In the dark, other objects may be seen because of the scattered light but this is very low level. In a lit room the background lighting will drown this effect. All you will see is the incident and reflected beam paths.

 

[Dale]

if I do the same experiment in outer space, where there is no scattering, would I still see the glow of the laser in the mirror?

Would you be able to see the reflection of your hand in a mirror in space?

You are talking about lasers and smoke. None of that is relevant to your actual confusion, IMO.

 

[elou]

Would you be able to see the reflection of your hand in a mirror in space?

I don't know. Do you?

 

[Ibix]

I don't know.

I'm really interested in why you think the answer might be "no". (It's "yes", by the way, assuming there's a light source like a star around.)

 

[jbriggs444]

I don't know. Do you?

Yes, @Dale does. He knows whether you could see your hand for a whole host of scenarios that match the description "hand in a mirror in space".

His question was an invitation for you to think. And to think out loud so that we can troubleshoot your reasoning.

Feel free to specify any missing details:

Is there a line of sight with a valid reflection angle from hand to eye?
Does this mean that there is necessarily also a line of sight from eye to hand?
Is the hand illuminated? Illuminated on the portion that faces the line of sight?
How do you think seeing works?

 

[Ibix]

It's "yes", by the way, assuming there's a light source like a star around.

Evidence:

Notice all the reflections in the visor.

Source

 

[Dale]

I don't know. Do you?

As @jbriggs444 said, I was intending for you to put your thoughts down.

Why is there any doubt in your mind as to this answer? Please explain your reasoning about why this is at all confusing.

(As others have said, the answer is a clear and unambiguous yes.)

 

[elou]

Yes, @Dale does. He knows whether you could see your hand for a whole host of scenarios that match the description "hand in a mirror in space".

His question was an invitation for you to think. And to think out loud so that we can troubleshoot your reasoning.

Feel free to specify any missing details:

Is there a line of sight with a valid reflection angle from hand to eye?
Does this mean that there is necessarily also a line of sight from eye to hand?
Is the hand illuminated? Illuminated on the portion that faces the line of sight?
How do you think seeing works?

Very kind of you to point at all the things I should worry about. The question was not what happens when things get hit by a beam, but what scattering means in outer space. If you could tell me what is being scattered in outer space and how that relates to the reflection of the laser head on the mirror, we can end this discussion with me much wiser than I started.
If we are looking not at something being illuminated, but at its reflection on a mirror, what is exactly happening that makes the mirror reflect the object, and us see the reflection. That is in short, what I would like to understand.

 

[Ibix]

what scattering means in outer space.

Same as it means anywhere else. Light hits a particle or something and some of the energy reflects off it. When the reflection isn't in one direction because the surface light is reflecting off isn't flat and smooth, we call it scattering.

how that relates to the reflection of the laser head on the mirror

Completely unrelated, unless you start spraying smoke. Any scattering off the few bits of space dust the beam might encounter are pretty much negligible.

If we are looking not at something being illuminated, but at its reflection on a mirror, what is exactly happening that makes the mirror reflect the object, and us see the reflection. That is in short, what I would like to understand.

Light comes out of a light source and hits an object, which scatters it in all directions (note that some materials reflect directionally, like mirrors, and most materials do a bit of both). Some of the scattered light might hit a mirror, and bounces off that. If it reaches your eye, you see it.

 

[elou]

Same as it means anywhere else. Light hits a particle or something and some of the energy reflects off it. When the reflection isn't in one direction because the surface light is reflecting off isn't flat and smooth, we call it scattering.

Completely unrelated, unless you start spraying smoke. Any scattering off the few bits of space dust the beam might encounter are pretty much negligible.

Light comes out of a light source and hits an object, which scatters it in all directions (note that some materials reflect directionally, like mirrors, and most materials do a bit of both). Some of the scattered light might hit a mirror, and bounces off that. If it reaches your eye, you see it.

Yes , of course. And because there is no way to make the scattering visible, we just have to take it in good faith.

 

[Ibix]

And because there is no way to make the scattering visible

Which scattering? The scattering off the illuminated object? How else do you think we see it?

 

[elou]

Which scattering? The scattering off the illuminated object? How else do you think we see it?

I don't know.