Is light visible ?

[Antony-Jones]

Help, please.

 

I read somewhere recently (I've forgotten, unfortunately) that light was actually invisible.

 

The reasoning was a thought-experiment. Imagine that you're on a spacecraft in deep space. You're in the cock-pit, looking out. There's a powerful search-light on top of the craft, currently off, pointing in the direction you're looking,

 

You look out at the scene, and then you turn on the light. Does the scene change ?

 

The book suggested that it wouldn't. The beam of light itself would be invisible, you wouldn't be able to see it, and in fact you wouldn't be able tell whether the light was actually operating.

 

Is this correct ?

 

If so, what does it mean ?

[Klaynos]

you only 'see' or detect the photons (little packets of light waves) that actually hit your eyes, in the search light example they are fired out of the search light away from you, so you could only detect them if they reflect off something else. No light reflects off of the photos so you cannot see them from that.

[Cap'n Refsmmat]

The reason a beam of light (from a searchlight, for example) is visible is because it is reflected off of particles in the atmosphere back to your eyes. In space, there are no particles to reflect the light, and the beam continues until it hits a solid object and can reflect.

[Antony-Jones]

hi guys, thanks,

 

OK, I'm getting there; I switch on the light, which initiates a stream of photons that I can't see because they're travelling away from me and so not going into my eye. So they're invisible to me.

 

But, if I can get the photons to go in my eye, either by bouncing them off an object, or putting my eyes directly into the beam, I will see them. They'll be visible to me.

 

Light is visible if goes in my eye, otherwise it's invisible, is that right ?

 

That seems obvious and weird at the same time.

[Klaynos]

It's not so much that it's invisible, but it is the thing we use to see with.

 

A good thing would be to take a torch outside on a rainy night, you can see the light reflecting off of the water droplets. But on a dry night you can't see the beam at all.

[Antony-Jones]

OK, another question....

 

I seem to remember the thought-experiment in the book continued thus...

 

You've turned on the first searchlight, but you can't tell whether it's on or not because you can't see it.

 

So you do what you normally do when you want to see something - you shine a light on it. You turn on another searchlight below the spacecraft to intersect with the first beam in front of you.

 

What happens, if anything ? Does the scene change ?

 

Can you see the first beam in the light of the second ?

[Cap'n Refsmmat]

No. Light does not reflect off of light. Remember, the light has to reflect back to your eyes for you to see it.

[Antony-Jones]

Light does not reflect off of light

 

Fair enough.

 

Do you know why it doesn't ?

 

 

 

 

And.. er.. doesn't this mean...

 

If you shone a light on the Invisible Man, you couldn't see him. Which is why he was The Invisible Man.

 

So, iIf you shine a light on light, but you still can't see it.... doesn't that make light The Invisible Light ?

[Cap'n Refsmmat]

Essentially.

 

Light does not reflect off of light because photons are massless, so there's really nothing to reflect off of.

 

But please ask a real physicist, because I'm probably lying to you.

[Sisyphus]

To put it in a slightly different way from other people:

 

Light is the only thing which IS visible. The only thing you ever see is the light which hits your retinas. When you "see" an object, you're not seeing the object itself, you're seeing the light, from some other source*, which reflects off of that object into your eye. That's why you can't see in the dark. You wouldn't see the spotlight because those photons aren't entering your eyes.

 

*Or sometimes the object itself is the source, if it's hot enough to give off visible light. Still, though, you're seeing the photons it gives off, not the thing itself.

 

EDIT: On why light doesn't reflect other light. Light travels as waves, which cannot alter one another's path. It's not special to light - you can see it with water waves. They just pass right through one another. Reflection can only occur at some hard surface, or when passing from one medium to another (like air and water).

[Antony-Jones]

"But please ask a real physicist, because I'm probably lying to you."

 

 

You fraud.

 

You will be hearing from my solicitors.

 

But in the meantime, have another question :

 

The thought experiment continues. You're on your spaceship returning to Earth, when you accidentally hit the "Planetary Destruct" button instead of the retro-rockets, thereby wiping out all life on Earth.

 

Assume no life exists elsewhere in the universe.

 

And sooner or later, you die too.

 

(Sorry, this isn't the most fun thought experiment in town, is it ?)

 

At the precise moment of your expiry, what happens ?

 

There are no longer any eyes for photons to go into.

 

So, does the scene change ?

 

Does the universe become invisible ? In an instant ?

 

In what sense could it be visible, if there are no eyes ?

 

What do you reckon, Cap'n ?

 

Ant.

[Cap'n Refsmmat]

That's like asking "If a tree falls down in a forest, and nobody's around to hear it, does it make a sound?"

[Antony-Jones]

To put it in a slightly different way from other people:

 

Light is the only thing which IS visible.

 

now that's funny, sisyphus.

 

Immediately after I've concluded that light is invisible, your "slightly different way" of putting it is that light is the only thing that is visible.

 

And yet, your comment also makes sense.

 

I'm beginning to conclude that the nature of light is somewhat contradictory.

[Antony-Jones]

cap'n said:

 

That's like asking "If a tree falls down in a forest, and nobody's around to hear it, does it make a sound?"

 

 

Yes indeed, that's the question, extrapolated to include every sense, not just sound.

 

Do you happen to know the answer to it ?

[Sisyphus]

 

Do you happen to know the answer to it ?

 

It's a question that's supposed to be contemplated but not answered. It doesn't have an answer.

[Klaynos]

That's like asking "If a tree falls down in a forest, and nobody's around to hear it, does it make a sound?"

 

Yes it does. If it doesn't all our laws of physics are wrong and we should probably just set fire to the philosophers for making it so.

 

 

Why do photons not reflect photons:

 

Firstly you need to understand how an atom reflects light:

 

http://www.sciam.com/askexpert_question.cfm?articleID=00054D50-1685-1514-968583414B7F0138&catID=3&topicID=13 (this is a somewhat simplified view)

 

But if we consider photons there are no electrons there to oscillate to absorb and then emitte a photon going in the opposite direction. So no reflection.

[Sisyphus]

Yes it does. If it doesn't all our laws of physics are wrong and we should probably just set fire to the philosophers for making it so.

 

Well, we need our scientists to behave as if they believe it does, yes. But if you consider that ultimately, all experiences are thoughts, and hence the life of the mind is the only real object of inquiry - :::notices torches, runs away:::

[Antony-Jones]

Yes it does. If it doesn't all our laws of physics are wrong

 

I don't understand , can you explain what you mean there, Klaynos ?

 

How would "no it doesn't" contradict the laws of physics ?

 

 

 

And thanks for the links, haven't had a chance to look at them yet.

 

And thanks to you and the others for being patient with a non-scientist's stupid questions....

 

Ant.

[Sisyphus]

How would "no it doesn't" contradict the laws of physics ?

 

Because they describe the objects acting causally in specified ways, wherein whether anyone is aware of these actions plays no role.

[Antony-Jones]

It seems to me that the question is highly ambiguous, because the word 'sound' has two very different but common definitions; the definition you use affects the answer you get.

 

from dictionary.com :

 

sound:

 

1. the sensation produced by stimulation of the organs of hearing by vibrations transmitted through the air or other medium.

 

2. mechanical vibrations transmitted through an elastic medium...

 

So Sisyphus, you and Klaynos are using the second definition, aren't you, which means that the tree hitting the ground causes vibrations in a medium, and that this happens irrespective of observers ? So the answer is 'yes, it does'.

 

Whereas if one chooses to use the first definition, that sound is an sensation, (requiring the existence of an organism using sense organs to experience the sensation), one can truthfully say that the falling tree does not cause a sound, because the question specifies that there are no auditory sense-organs (I could have said 'ears') within range (earshot) of the physical waves, and therefore no sensations.

 

So yes are and no are both correct, depending on what you mean by 'sound'.

 

 

Does that make sense ?

[insane_alien]

Antony, we are scientists and engineers. we use the scientific definition rather than rely on the dictionary meanings of words. this cuts out the possibilities of misunderstanding something. the scientific definition is the second one. a mechanical vibration. in this case the medium is air.

[Antony-Jones]

hi Alien,

 

I'm not sure what you mean.

 

You say :

 

we use the scientific definition rather than rely on the dictionary meanings of words.

 

but then you admit :

 

the scientific definition is the second one

 

ie What you call the 'scientific' definition, I found in the dictionary.

 

So it can't be a matter of choosing between the one or the other, can it ? One contains the other.

 

Secondly, I don't agree that only the second definition is 'scientific'.

 

The first concerns sensation and hearing, (as well as sound-waves). It is therefore the subject of scientific enquiry in a number of fields, such as biology, psychology, neuro-biology, anatomy and acoustics.

 

 

 

Ant.

[Klaynos]

But you're posting in a physics section, the physical meaning is not sensory.

[insane_alien]

well, the dictionary does occasionally list the scientific meaning along with other nonscientific meanings. the scientific definition of sound is a mechanical vibration in an elastic medium. the sensory part is not a soundwave therefore should not be included in the definition. just as we do not describe photoresistors in our definition of light because they can detect light.

 

and as klay said, this is the physics forum.

[grifter]

I understand most of the high-level concepts of physics on this forum, yet, not why Antony-Jones posts the same questions re-worded, or even a different question, with little relevance. so, even if what I'm about to post makes no sense, I must ask that Mr Jones please accepts it, for the good of humanity.

 

light can be seen as a wave or as a particle, that particle being the photon.

 

"light" (be it waves or particles.) exists even when one cannot see it.

 

"light" can only be seen by the naked eye when real "pieces" of light (photons) hit the retina.

 

to understand why light cannot "bounce off" light you must think of it the light as a wave...... as waves have no mass, hence no surface area, there is nothing for the light to "bounce" off.

 

this is, to quite a large degree, simplified, in the hope that it at least enables you to see the basic concepts of how "light" works.

 

QED(ish)