A theory of why light waves are (or aren't) invisible ?

deckstunt

I remember hearing a scientist (or commentator) say that light is invisible and that it was only when light was reflected from an object that it became visible. So for example in space, say on the International Space Station, when looking out into the blackness of space, that 'blackness' is actually filled with light waves from the Sun, but they are invisible!

That intrigued me, that light itself was 'invisible' and it got me thinking. I think I understand the reason behind this. Could someone who knows, confirm or refute my theory:

A light wave is only visible when it enters the eye perpendicular to the retina (sounds obvious, but bear with me) so that the electric field and the magnetic field of the light wave would form a +. (In reference to the illustration below of a light wave at kind of 45 degrees view. Imagine looking straight down the 'arrow' and the wave would become a +)



But when a light wave is viewed sideways-on, i.e. like a sine wave ~, it is not visible.

So, when an astronaut on the ISS is looking into the blackness of space, even though that space is filled with light waves from the sun he is looking at them sideways-on. And as a result they are invisible.

If he was to look into the sun (if he wasn't blinded first) he would be looking at the light wave head on and the light wave becomes visible. (i.e. like a "+". The electric filed and the magnetic field making up the vertical and horizontal arms of the +)

Or similarly if the astronaut saw an object illuminated by the sun, say the Space Shuttle, the reason he 'sees' the Shuttle is because the reflected light waves are entering perpendicular to his retina, head-on in that + .

So is it correct to refine the original claim (that light is invisible and that it was only when light was reflected from an object that it became visible) to state that a light wave is only visible when viewed head-on. That it is actually the plane that we view the light wave from is what matters and not per say any object reflecting the light wave?

seamus

I think it's simpler than you're making it out to be.

Light is only detectable when it interacts with a photoreceptor. Something is only visible because the light it emits or reflects, enters the eye and stimulates the retina. If the light doesn't stimulate the retina, it's not detectable.

To say that something is "invisible" is to say that light waves pass through it without altering the wave. Which would be true of light itself.

Think of it in terms of sound. Sound is inaudible, except when it enters your ear

deckstunt

seamus wrote: »

I think it's simpler than you're making it out to be.

Light is only detectable when it interacts with a photoreceptor. Something is only visible because the light it emits or reflects, enters the eye and stimulates the retina.

Bugger!

Seamus, I think you're right.

Thanks.:o

rccaulfield

seamus wrote: »

I think it's simpler than you're making it out to be.

Light is only detectable when it interacts with a photoreceptor. Something is only visible because the light it emits or reflects, enters the eye and stimulates the retina. If the light doesn't stimulate the retina, it's not detectable.

To say that something is "invisible" is to say that light waves pass through it without altering the wave. Which would be true of light itself.

Think of it in terms of sound. Sound is inaudible, except when it enters your ear

I don't agrre with that, are you saying the old chestnut- if a tree fell in the woods while only plants were on the land then it made no sound?

SOL

No, I think he's saying that if your ear doesn't interact with the sound wave then it wont hear it. The sound wave exists either way, but you can only hear it when your ear absorbs energy from it...

seamus

rccaulfield wrote: »

I don't agrre with that, are you saying the old chestnut- if a tree fell in the woods while only plants were on the land then it made no sound?

No, absolutely not. But if the sound waves don't hit your eardrum, then you would say that it's inaudible. You would know that there is sound there, but you can't hear it. Just like you know there are light waves coming from the sun, but you can't see them.

I think I can actually simplify my response a little more. Something is visible only because it emits or reflects light. Light waves do not emit or reflect light

Improbable

Am I right in saying that the electric and magnetic fields oscillate perpendicular to the direction of propagation of a light wave? So in essence, the OP is right in that we can only "see" when the light making that "+" as if you were viewing the light wave head-on so to speak. If the light is entering your eye, the oscillations MUST making that "sign". So I think he's kind of right, though not for the right reasons.

seamus

He is right, I think, primarily in terms that light is travelling in a single direction and only becomes "visible" when the viewer stands in its way. When you stand side-on (or indeed behind it), the light wave does not interact with the detector.

I think it probably comes down to the nature of how the eye picks up light. I imagine the retinal cells will respond to light which hits them from *any* direction, but the eye is constructed to only allow light to enter from a limited set of angles, so in general the light hits the retinal cells from the "front" (as we know it). That is, light rays travelling perpendicular to the observer are not detected because they do not interact with the retinal cells - your eyes are white because the eye wall reflects all other ambient light to prevent interference with the retina.

As I say, I think he's making it a little more complicated than it needs to be. Light is only visible when it's moving towards you for the same reason that a moving car only hits you when you stand in front of it.