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Author Topic: What is the carrying capacity of a photon?  (Read 2860 times)

Offline fireng

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What is the carrying capacity of a photon?
« on: 06/02/2013 07:07:34 »
When observing anything, the image is brought to us by photons - right?
Does each photon carry the entire picture?
How does our eyes ( Or photographic film) decode this?
« Last Edit: 06/02/2013 12:45:09 by JP »


 

Offline CliffordK

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Re: carrying capacity of a photon.
« Reply #1 on: 06/02/2013 07:43:29 »
Yes, the eye reacts to photons.
No, a single photon doesn't carry the whole image.

The human eye is somewhat like a digital camera.  The eye has about 120 million rod cells (Black and White) and 6 million cone cells (Color). 

So, your eyeball is essentially equivalent to a 126 megapixel camera.

If a photon strikes a rod cell, it will activate that cell and you will see essentially a single dot.  Cones, however, only respond to photons of the right wavelength.

Now, think of any dot being illuminated on your wall.  It sends light out in all directions, but only one direction leads to your eye. 

Likewise, another dot on the wall sends light out in all directions, but a unique direction leads to your eye. 

Now, if the photo sensors were just laid out on the skin, then the directions would all get blurred.  However, since your eyes have a lens, it focuses all the light through the equivalent of a single point, and creates an exact reverse image of the light on the back of the eye corresponding to only those light vectors that lead through the middle of the eyeball.

So, by capturing different numbers of photons, and different wavelengths, you get the lightness and darkness, and various colors that make up the image that you see. 

The retina itself is an extension of the brain, and signal processing starts early including cell arrangements to pick up the signals from the rods and cones and determine lines, contrast, and movement.  This processing increases in complexity in the optical cortex and the rest of the brain.

Likewise, with an old film camera.  If you exposed the film to the sunlight without a camera and lens, everything would be out of focus, receiving light from all directions, and the film would wash out white.

The lens focuses light from each direction to a different spot on the film, which is then activated based on wavelength, intensity, and etc. 

A pin hole camera essentially functions the same as a lens, with the light being focused through a single hole.
 

Offline fireng

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Re: carrying capacity of a photon.
« Reply #2 on: 06/02/2013 08:34:54 »
So more photons equals a more complete picture ?  Thinking more about photons arriving from very far away eg big bang.
 

Offline CliffordK

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Re: carrying capacity of a photon.
« Reply #3 on: 06/02/2013 08:57:03 »
Yes,
The more photons, the more complete the picture.

A telescope takes light from say one point of origin, that then lands on a large mirror, then focuses this back down to a single spot on the camera, or eye.  And, thus, can consolidate more light.  The bigger the telescope, the more light it can consolidate.  The telescope can also consolidate the light over time. 

So, say you're looking at a distant star with your eyes, say with a 3mm aperture, and expecting, say on the order of 60 photons a second to make a "picture".  It also adjusts is sensitivity, so if you have brighter closer stars, they drown out the dim distant stars.

A large telescope, rather than picking up light from a 3mm opening, it picks up light on, say a 3 meter area, or a thousand times (in one direction) larger area, or a total of 10002, or a million times larger area than your eye.  Then, assuming tracking capabilities, it can sum the light coming in over several hours, or perhaps even days, whereas your eye is expecting a continuous stream of photons to build and maintain the image.  Just an occasional photon (in a dark room) will just appear as a dim flash.

Thus, even a few very rare photons from a distant star could be significant.

Keep in mind that the star generally emits light in all directions.  So, the intensity (number of photons per area) falls off with the square of the distance.  A star 20 lightyears away will be as bright as one 10 lightyears away.  Thus, as many photons reaching the eye per unit time.

P.S.
If this is a physiology question as it first appeared, then is should be under physiology.
If, however, you were intending an astronomy question, then I can move it back to Physics & astronomy.
 

Offline yor_on

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Re: What is the carrying capacity of a photon?
« Reply #4 on: 07/02/2013 08:11:10 »
Fireng, if you think of it as waves Cliffords explanation makes eminent sense. But considering it strictly as photons each photon has a momentum, a quantized energy and a 'spin/polarization'. And to create a picture from that I'm not sure how to do? Maybe it's possible though?

You do have a constant duality (waves/photons) to consider, but then the question becomes how photons becomes waves, and presumably vice versa. It could have to do with how the eye is constructed, thinking of how you can use those classical slot experiments to 'force' photons into waves, but it's a little to late, or early, for me :)

But I've been wondering about it too.
 

Offline evan_au

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Re: What is the carrying capacity of a photon? (...in bits)
« Reply #5 on: 08/02/2013 10:38:23 »
Looking at the question from another viewpoint: How much information can a photon carry? (measured in bits)
This is a question of great interest in telecommunications.

In older optical fiber systems, information was transmitted by turning a laser on and off very quickly. A burst of (say) 100,000 photons at the transmitter would represent a "1", and < 1000 photons in the same time would represent a "0". However, 99.9% of these photons would be lost by traveling through many miles of optical fiber, so at the receiver, a burst of about 100 photons represents a "1", and a much smaller number of photons represents a "0". So you could say that it took about 100 photons to reliably represent 1 bit of information.

However, there are more complex ways of imposing information on a photon, by controlling its:
  • Polarisation
  • Phase: horizontal/vertical/clockwise/anticlockwise
  • Frequency/wavelength
  • Transmission mode, in environments where multiple modes are possible
  • Together, these could potentially encode 8 or more bits into a single photon.
  • ...even without considering the possibility of transmission of quantum states by means of entangled photons! (an area of advanced research)

However, all of these methods have to deal with the fact that with today's optical fibres, <1% of the photons will reach the far end (unless the distance is extremely short). This means that most of the information will be lost in transit. This is usually handled by encoding a group of photons with one or more of the advanced methods above, and then applying an error-correcting code which will reconstruct the information which is lost in transit (provided the losses are not too great).

So even with the best available techniques today, we are not able to carry more than 1 bit per photon over a long distance.
 

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Re: What is the carrying capacity of a photon? (...in bits)
« Reply #5 on: 08/02/2013 10:38:23 »

 

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