Light beams and their reflection

[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.

 

[A.T.]

I don't know.

If you can't think of an alternative to scattering to explain how we see illuminated objects, then why do you question that it is scattering?

 

[elou]

If you can't think of an alternative to scattering to explain how we see illuminated objects, then why do you question that it is scattering?

I am, rightly or wrongly, not convinced. I was hoping to find a clear answer one way or another.

 

[jbriggs444]

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.

A mirror does "specular reflection". Most ordinary objects (trees, grass, sand, concrete, wood, birds, hands, etc) do "diffuse reflection". As @Ibix tried to say without using the big words.

In specular reflection, an incident ray is all reflected in a single direction. You know: angle of reflection equals angle of incidence. This is characteristic of smooth surfaces. In terms of wave optics (think Huygen's principle), the ray path is one that extremizes path length, at least locally. Importantly, this means that the first derivitive of path length with respect to impact location is zero. Small deviations from the center of line result in wave forms that interfere constructively.

By contrast, microscopially rough surfaces tend to reflect diffusely. Small deviations from the center line result in significant variations in path length. There is no constructive interference.

With specular reflection, nearly equal angles of incidence yield nearly equal angles of reflection. A beam of light is all reflected the same way. The angle of reflection yields information on the location of the source.

With diffuse reflection, the angle of reflection tells us nothing about the angle of incidence. The location of the illumination source is not determinable by observing the diffusely reflected light.

We "see" a point on an illuminated object because the portion of the light that is emitted in a very narrow fan-shaped arc stays aligned with the rest of that fan-shaped spray all the way to our eye. The lens of the eye is then able to focus all of that incident light at a single point on the retina that corresponds to the angle of incidence on the eye. This yields what might be seen as a two-dimensional hemispherical bit map of the light incident on the eye. The rods and cones in the eye corresponding to bits in the bit map.

Post-processing in the eye and the visual cortex mean that the bit map metaphor is not technically apt. The sensing bioware is surprisingly complex. But it is good enough for purposes of the ray optics we are discussing.

 

[elou]

A mirror does "specular reflection". Most ordinary objects (trees, grass, sand, concrete, wood, birds, hands, etc) do "diffuse reflection". As @Ibix tried to say without using the big words.

In specular reflection, an incident ray is all reflected in a single direction. You know: angle of reflection equals angle of incidence. This is characteristic of smooth surfaces. In terms of wave optics (think Huygen's principle), the ray path is one that extremizes path length, at least locally. Importantly, this means that the first derivitive of path length with respect to impact location is zero. Small deviations from the center of line result in wave forms that interfere constructively.

By contrast, microscopially rough surfaces tend to reflect diffusely. Small deviations from the center line result in significant variations in path length. There is no constructive interference.

With specular reflection, nearly equal angles of incidence yield nearly equal angles of reflection. A beam of light is all reflected the same way. The angle of reflection yields information on the location of the source.

With diffuse reflection, the angle of reflection tells us nothing about the angle of incidence. The location of the illumination source is not determinable by observing the diffusely reflected light.

We "see" a point on an illuminated object because the portion of the light that is emitted in a very narrow fan-shaped arc stays aligned with the rest of that fan-shaped spray all the way to our eye. The lens of the eye is then able to focus all of that incident light at a single point on the retina that corresponds to the angle of incidence on the eye. This yields what might be seen as a two-dimensional hemispherical bit map of the light incident on the eye.

Post-processing in the eye and the visual cortex mean that the bit map metaphor is not technically apt. But it is good enough for purposes of the ray optics we are discussing.

A very nice summary of reflection phenomena.

 

[weirdoguy]

not convinced.

I have a feeling you will never be convinced.

 

[sophiecentaur]

I am, rightly or wrongly, not convinced. I was hoping to find a clear answer one way or another.

I have a feeling that you are demanding 'spoon feeding' about this. If you put a postage stamp to cover the beam, all the light will be reflected / blocked and you will see a red disc and there will be a shadow behind it. If you put a grain of rice in the beam, it will block / reflect some / most of the beam. If you put a speck of dust, you will see it because it reflects a small proportion of the beam. For very small items, the light may be scattered in all directions due to diffraction.

Google "Scattering of Light" and read around the topic. Put in a bit of your own effort and figure out why you are not getting this. I think you just like all the attention you are getting from PF.

 

[Dale]

what is exactly happening that makes the mirror reflect the object, and us see the reflection

Our eyes see light that enters the eye.

If you are in a dark room you cannot see anything because there is no light to enter your eye from anything.

If you turn on a light bulb in the room then light from the bulb hits your hand and scatters in all directions. One of those directions is from your hand to your eye. That light enters your eye and you see your hand.

If there is a mirror in the room then there are two directions from your hand to your eye. The direct one described above, and one that reflects off the mirror. So light can reach your eye by scattering off your hand in two directions. You can see the back of your hand directly (for example) while light from the palm of your hand scatters off the palm in all directions including the one that hits the mirror and reflects to your eye.

 

[sophiecentaur]

Our eyes see light that enters the eye.

If you are in a dark room you cannot see anything because there is no light to enter your eye from anything.

If you turn on a light bulb in the room then light from the bulb hits your hand and scatters in all directions. One of those directions is from your hand to your eye. That light enters your eye and you see your hand.

If there is a mirror in the room then there are two directions from your hand to your eye. The direct one described above, and one that reflects off the mirror. So light can reach your eye by scattering off your hand in two directions. You can see the back of your hand directly (for example) while light from the palm of your hand scatters off the palm in all directions including the one that hits the mirror and reflects to your eye.

You can lead a horse to water. . . . .

 

[A.T.]

I was hoping to find a clear answer one way or another.

Then clearly explain what is still unclear.

 

[Vanadium 50]

You can lead a horse to water. . . . .

... but you can't make him think.

 

[elou]

As far as I am concerned, this thread can be closed.

 

[sophiecentaur]

That was the “horticulture” saying, iirc.

 

[Vanadium 50]

The problem seems to be not that @elou doesn't understand our explanations, it is that he doesn't believe them. Obviously, people can believe whatever they wish. That doesn't mean they can call it science.

I think at this point, the best advice is that he spend the $5 on a cheap laser pointer and see whatever what he has been told matches observation or not.

 

[gmax137]

Here's another thing to explain. The image in a mirror is reversed, left to right, right? Why is it not also reversed top to bottom? Is it an effect of gravity? Will it be different in outer space? I think if @elou can work this out he will be on his way to answering the laser question.

 

[Dale]

As far as I am concerned, this thread can be closed.

I am glad we could help.