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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: neilep on 20/10/2007 17:11:09

Title: Intersecting Beams Of Light Question ?
Post by: neilep on 20/10/2007 17:11:09
Dear Light Experts and Lighthouse Keepers,

Aren't these beams of light beautiful?

[ Invalid Attachment ]

IF I was to shine a red light beam through a yellow light beam would the yellow and read beams at the point of crossing  turn each other orange ?...and what colour would the light beams be once they leave the intersection ?...

can you explain why ?

Title: Intersecting Beams Of Light Question ?
Post by: Karen W. on 20/10/2007 17:15:39
That seems like what would happen but I don't know for sure.

Would they all end up together after the crossing to make a white light??? Maybe that would be my guess..
Title: Intersecting Beams Of Light Question ?
Post by: neilep on 20/10/2007 17:25:18
That seems like what would happen but I don't know for sure.

Would they all end up together after the crossing to make a white light??? Maybe that would be my guess..

This is kind of what I mean !!

[diagram=286_0]
Title: Intersecting Beams Of Light Question ?
Post by: neilep on 20/10/2007 17:25:52
erhmmm...the orange bit was much better than that in the piccy I drew !!
Title: Intersecting Beams Of Light Question ?
Post by: another_someone on 20/10/2007 18:16:14
What do you mean by 'turn each other orange'.

Light has no colour - it is our perception of the light that has colour (hence why there can be differences in the perception of colour).  If we turn one way, we will see one beam, but if we turn another way, we will not see that beam but will see the other beam, so we would never perceive the two beams mixed.
Title: Intersecting Beams Of Light Question ?
Post by: neilep on 20/10/2007 18:31:47
What do you mean by 'turn each other orange'.

Light has no colour - it is our perception of the light that has colour (hence why there can be differences in the perception of colour).  If we turn one way, we will see one beam, but if we turn another way, we will not see that beam but will see the other beam, so we would never perceive the two beams mixed.

Well...put it like this...we both have torches.....yours has a red light bulb in it...mine has a yellow and we criss cross the beams like in the picture above.....surely we'll be able to identify each others beams of light won;t we ?....so...when they criss cross....what will be the colour of the light at the moment of intersection ? (when the two light beams touch each other?)....and what will be the colour of the beams as they leave the intersection ?
Title: Intersecting Beams Of Light Question ?
Post by: another_someone on 20/10/2007 18:42:51
Well...put it like this...we both have torches.....yours has a red light bulb in it...mine has a yellow and we criss cross the beams like in the picture above.....surely we'll be able to identify each others beams of light won;t we ?....so...when they criss cross....what will be the colour of the light at the moment of intersection ? (when the two light beams touch each other?)....and what will be the colour of the beams as they leave the intersection ?

Put it like this - towards which torch will you be looking - if you look towards the red torch, you will see red; if you look towards the yellow torch, you will see yellow.

Have you never been in a situation where you see a light ahead of you in one colour, and another light in a different colour from the corner of your eyes?  Clearly, the light from the two light sources must both be reaching your eyes, and so must be crossing each others path at around the point of your eye, yet do you not still discern two separate sources of light, with two separate colours, rather than having the colour blended into one.

The only time you will perceive the colours to be blended is not when the light sources are widely separated, but when they are closely juxtaposed, and too close for your eyes to resolve the separation (as in the dots in a TV screen).

The only other case where you might see the colours being blended is if you pass the light through some smoke (or some other medium that will scatter the light), but in that case, although the scattered light will highlight where the beems travel, you are not actually seeing the beams themselves, but the light scattered off the particulates in the smoke (which is non-direction, and hence the both colours will appear to be coming from the same direction).
Title: Intersecting Beams Of Light Question ?
Post by: neilep on 20/10/2007 19:24:57
Well...put it like this...we both have torches.....yours has a red light bulb in it...mine has a yellow and we criss cross the beams like in the picture above.....surely we'll be able to identify each others beams of light won;t we ?....so...when they criss cross....what will be the colour of the light at the moment of intersection ? (when the two light beams touch each other?)....and what will be the colour of the beams as they leave the intersection ?

Put it like this -CHEEKY !! towards which torch will you be looking - if you look towards the red torch, you will see red; if you look towards the yellow torch, you will see yellow.

Have you never been in a situation where you see a light ahead of you in one colour, and another light in a different colour from the corner of your eyes?  Clearly, the light from the two light sources must both be reaching your eyes, and so must be crossing each others path at around the point of your eye, yet do you not still discern two separate sources of light, with two separate colours, rather than having the colour blended into one.

The only time you will perceive the colours to be blended is not when the light sources are widely separated, but when they are closely juxtaposed, and too close for your eyes to resolve the separation (as in the dots in a TV screen).

The only other case where you might see the colours being blended is if you pass the light through some smoke (or some other medium that will scatter the light), but in that case, although the scattered light will highlight where the beems travel, you are not actually seeing the beams themselves, but the light scattered off the particulates in the smoke (which is non-direction, and hence the both colours will appear to be coming from the same direction).

I see !!... ([;D]).....so.....my question is null and void because where ever I look I am seeing different beams of light which are obviously criss crossing each other anyway !!

Thank You George for *cough cough* helping me see the light .......*groan [;D]*
Title: Intersecting Beams Of Light Question ?
Post by: McQueen on 21/10/2007 07:34:29
Quote
I see !!... (grin).....so.....my question is null and void because where ever I look I am seeing different beams of light which are obviously criss crossing each other anyway !!
OR this is another example of wave particle duality.
Title: Intersecting Beams Of Light Question ?
Post by: daveshorts on 21/10/2007 14:56:52
In a vacuum which (air approximates to pretty well) light beams don't interact, so if you cross a yellow light a red one you will have some red light going in one direction and some yellow light going in the other. If this was not the case it would be almost impossible to interpret what the light that got to your eyes meant as it could have been changed several times on the way to your eyes.

In fact the observation that mixing red light and yellow light to make orange is purely an artefact of your rather limited colour vision approximating a mixture of an infinite number of dfferent frequencies to an amount of reddish, blueish and greenish lights.
http://www.thenakedscientists.com/HTML/kitchen-science/exp/why-sodium-street-lights-make-things-look-orange/
Title: Intersecting Beams Of Light Question ?
Post by: neilep on 21/10/2007 16:02:53
FANTASTIC DAVE...

Thank you...

a little bird tells me that(regarding the link you posted)........ the same can be done with a cd/dvd instead of the magazine too.....

...and then I found this in the kitchen science thread  http://www.thenakedscientists.com/forum/index.php?topic=6730.msg71859#msg71859

THANK YOU DAVE....
Title: Intersecting Beams Of Light Question ?
Post by: Karen W. on 21/10/2007 16:28:58
That seems like what would happen but I don't know for sure.

Would they all end up together after the crossing to make a white light??? Maybe that would be my guess..

This is kind of what I mean !!

[diagram=286_0]

Well I get this part, but after that is where I get lost! is it dispersed or what..?

George what you said here I don't understand ..

What do you mean by 'turn each other orange'.

Light has no colour - it is our perception of the light that has colour (hence why there can be differences in the perception of colour).  If we turn one way, we will see one beam, but if we turn another way, we will not see that beam but will see the other beam, so we would never perceive the two beams mixed.
Title: Intersecting Beams Of Light Question ?
Post by: lightarrow on 21/10/2007 18:06:05
Dear Light Experts and Lighthouse Keepers,

Aren't these beams of light beautiful?

[ Invalid Attachment ]

IF I was to shine a red light beam through a yellow light beam would the yellow and read beams at the point of crossing  turn each other orange ?...and what colour would the light beams be once they leave the intersection ?...

can you explain why ?

Your question implies a non-linear kind of interaction between light beams; this is possible but inside matter, not in the void; in the void could be possible, theoretically, at incredible high energies (as intensities or as frequencies), but I think it hasn't been proved experimentally; in any case, I have no idea of what could happen in this last case.
Title: Intersecting Beams Of Light Question ?
Post by: lyner on 21/10/2007 23:11:37
Two waves of any kind - sound, electromag, mechanical etc, will produce what are called intermodulation products when there is a non-linearity involved.  The products are a bit like harmonics but they occur at frequencies that are sums and differences of multiples of the original frequencies. In the case of light, the intermodulation products would very likely lie outside the visible range. (This is because our visible range covers only an octave frequency range).
To achieve the effect, you need high intensity laser light and 'certain' crystals. (Not the alternative medicine types, though!)
The effect is much easier to produce at lower frequencies - every type of radio receiver (Radio, TV, microwave) uses it to demodulate and detect the signals.  Guitar 'fuzz boxes do the same thing. You can even do it yourself, with sound.  Just whistle and hum at the same time, loudly and you will hear the 'beat' frequencies because your ears are not linear.
Title: Intersecting Beams Of Light Question ?
Post by: techmind on 21/10/2007 23:30:50
IF I was to shine a red light beam through a yellow light beam would the yellow and read beams at the point of crossing  turn each other orange ?...and what colour would the light beams be once they leave the intersection ?...

Others have pedantically (and quite correctly) said that you'll only "see" whichever beam you're looking in to. This would be correct for beams in vacuum or clear/clean air. However, if you had some theatrical smoke (smoke, or CO2 fog, or just steam from a kettle) then you could see both beams as per your diagram, and you should see an orange colour at the intersection.

Light is still predominantly forward-scattered so the relative intensity of the two beams (and hence the shade of orange) can still be expected to depend somewhat on your viewing-angle.

Assuming that the air (and fog) is "linear" (it will be for the purposes of this experiment) the two beams will superpose, but not interact... and will continue on their way (and with their original colours) after the crossing.
Title: Intersecting Beams Of Light Question ?
Post by: lyner on 22/10/2007 11:43:41
techmind; I think that is the real answer - we all piled into a technical discussion of   non linearity etc. In practice, you would get just what you describe. After all, you wouldn't be seeing the beams at all if there weren't some scattering.
Title: Intersecting Beams Of Light Question ?
Post by: lightarrow on 22/10/2007 13:42:07
techmind; I think that is the real answer - we all piled into a technical discussion of   non linearity etc. In practice, you would get just what you describe. After all, you wouldn't be seeing the beams at all if there weren't some scattering.
I quote techmind too; however neilep didn't talk about visible light beams.  [:)]
Title: Intersecting Beams Of Light Question ?
Post by: lyner on 22/10/2007 15:24:25
But the pictures imply that you can - the beams are shown form the side!
There was a Scientist - non Scientist communications clash, I think.