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Author Topic: What happens if we send a short pulse of white light through a prism?  (Read 2641 times)

Victor Nelson

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Victor Nelson asked the Naked Scientists:
   
We know a glass prism will separate a white light beam into its constituent monochromatic beams.  But we also know a monochromatic beam must be very long-in the limit, infinitely long for a perfectly precise frequency.

What if we send a short pulse of white light through a prism?

The emergent monochromatic beams, being very long, would have to be coming out of the prism long before, and long after, the short light pulse went through it.  That makes no sense.

Thank You,

V. Nelson

Frederick, MD                

What do you think?
« Last Edit: 28/02/2011 21:30:03 by _system »


 

Offline lightarrow

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Infact you won't have monochromatic rays coming out, but photons with not well defined frequency: if you put your tiny detector in a specific direction, you won't have photons hit it all the times the light pulse is sent through the prism.
 

Offline lightarrow

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Infact you won't have monochromatic rays coming out, you'll have photons with not well defined frequency: if you put your tiny detector in a specific direction, you won't have photons hit it all the times the light pulse is sent through the prism.
 

Offline yor_on

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A very nice question. Where will the break between monochromatic beams and spurious photons occur? And will they be sorted according to frequency's/energy, they should, shouldn't they (probability I mean)?
 

Offline lightarrow

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Where will the break between monochromatic beams and spurious photons occur?
Can you precise your question? 
Quote
And will they be sorted according to frequency's/energy, they should, shouldn't they (probability I mean)?
Yes, a precise frequency goes in a very precise direction; not well precise frequency photons will not all go in the same direction (= point hit in the screen).
 

Offline yor_on

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I was wondering if there was, I mean it seems it should be? A duration for when the output changes from continuous beams to singular 'photons' coming. Maybe it's not possible to define but imagine using your eye looking, at what duration does the monochromatic beams stop, and you get a 'flickering' effect instead. Also if it would have to do with the energy?
=

Ah forget about the energy :)
ahem..
==

That is, thinking of light as a wave, wondering at what stage the 'cutoff' will produce singular photons instead of a beam. The other way is to look at it as lightquanta, then the question becomes, at what duration do they turn into a wave/monochromatic beam :)
« Last Edit: 02/03/2011 18:18:59 by yor_on »
 

Offline lightarrow

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It depends on the beam intensity: greater intensity means more photons per unit of time hitting the screen; reduce enough the intensity and you have 1 photon every year  :)
 

Offline yor_on

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Intensity is the number of particles per area, am I right Lightarrow? How does that work with a wave? I've looked at that definition before and found it 'wanting', well as long as you don't define the universe to expand and contract relative/around each 'particle', I think?

I need to refresh myself on that one, but I know that the definition of particles coming in smaller or larger time-interval's (as waves) did not work out for me the last time I looked at it, exchanging waves for particles. And as I can do that 'instantly' actually allow that light to be in a 'superposition' between particles and waves, it should work out somehow? But I didn't find it doing so last time I looked?

But it might if we assume that a red and blue shift is a result of SpaceTime contracting relative the relation between the observer and what he observes, maybe? Sorry Lightarrow, I've just woke up :)
 

Offline yor_on

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Maybe it's stupid to try to reconcile waves with how photons is thought to behave? I'm not sure there, if we take the expansion then a explanation to the red-shift is how a wave gets 'stretched out' as new 'space' is formed in its 'propagation'. That may work for a wave, but for a photon? My argument against would be that you can't measure one 'photon' from that red shift, but then again, I think you can, and that you will find it red shifted.

There has to be a better way than to argue that the intensity is the number of particles/photons per time segment/area? Light gives me a headache.
 

Offline lightarrow

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Maybe it's stupid to try to reconcile waves with how photons is thought to behave? I'm not sure there, if we take the expansion then a explanation to the red-shift is how a wave gets 'stretched out' as new 'space' is formed in its 'propagation'. That may work for a wave, but for a photon? My argument against would be that you can't measure one 'photon' from that red shift, but then again, I think you can, and that you will find it red shifted.

There has to be a better way than to argue that the intensity is the number of particles/photons per time segment/area? Light gives me a headache.
Photons are not simple objects. Furthermore, the image of particles as localized and individualized corpuscles in space, is rapidly going towards is sunset...
What will take its place is the concept of "field". So you can more easily think about EM waves only, and your headache will finish forever  :)
Photons are just "quantized excitations of the field"; if you need an image you can think of "ripples" in the field.
The field interacts with matter, or with itself, and the result is a quantized "exchange of energy". That is the photon.
 

Offline lightarrow

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Intensity is the number of particles per area, am I right Lightarrow? How does that work with a wave?
Poynting vector: the vector product of electric field and magnetic field. For an EM wave in the void it's proportional to electric field square modulus.

Quote
But it might if we assume that a red and blue shift is a result of SpaceTime contracting relative the relation between the observer and what he observes, maybe? Sorry Lightarrow, I've just woke up :)
Does spacetime expands/contracts in the Doppler effect of Sound? It's a simple effect of waves, no expansion/contraction of spacetime.
« Last Edit: 04/03/2011 11:55:21 by lightarrow »
 

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