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Final Answers
© 2000-2015   Gérard P. Michon, Ph.D.

Photons
Quanta of Electromagnetic Radiation

Let there be light !
Genesis 1:3

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Wikipedia :   Photon   |   Photonics   |   Nanophotonics

 
Video:  What is a photonWhere do photons come from?  by  Steve Johnson.
How do we see light(Nobel 2012; Serge Haroche, David Wineland)  in  MinutePhysics.
 
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Photonics


tk3078  (Yahoo!  2010-12-29)   Studying the quanta of light.
What's the difference between  photonics  and optics?

Optics  deal with light in a classical way  (i.e, without quantum concepts)  using one of two viewpoints:

  • Geometrical optics  is based on the concept of  light rays  propagating in a straight line according to the classical laws of reflection  (angle of reflexion = angle of incidence)  and refraction  (Snell's law).  Both of these where unified as consequences of the  principle of least time  postulated by Pierre de Fermat iaround 1635 and confirmed experimentally in 1851  (when it was finally established that the celerity of light is indeed inversely proportional to the index of refraction of the medium).
     
  • The  wave theory of light,  on the other hand, explains diffraction  (as well as the laws of reflexion and refraction of geometrical optics, incidentally).  It was first championned by Christiaan Huygens and received experimental support from Thomas Young in 1803.  The idea that light is a form of electromagnetic wave is due to Michael Faraday, who was later vindicated mathematically by James Clerk Maxwell  (Maxwell's equations, 1864).

By contrast,  quantum optics  (fundamental research)  and  photonics  (applied science)  are based on the explicit idea that light consists of packets of energy proportional to its frequency  (the coefficient of proportionality being  Planck's constant).  This idea was formally put forth in 1905 by Albert Einstein to explain the photoelectric effect  (in 1900, Max Planck had paved the way by showing that the blackbody spectrum could be explained by postulating that  all energy exchanges  between radiation and matter could only occur in  quanta  of energy proportional to the frequency).

So, the key difference between optics and photonics is that the latter deals primarily with the  quantization of light  which is ignored by the former.

Also, in optics we consider light to consist  either  of particles  (explaining the  light rays  and  sharp shadows  on which geometrical optics is based)  or  waves  (which explain diffraction using Huygens principle).  In  photonics,  we integrate the quantum notion that the light quanta  (photons)  have properties characteristic of  both  waves and particles.

Wikipedia :   Photonics vs. Geometrical optics.

 Heinrich Hertz 
 (1743-1794)
(2011-01-03)   The Photoelectric Effect  (Einstein, 1905)
What is the  work function  of a metal?

The photoelectric effect was first observed in 1887, by  Heinrich Hertz (1857-1894).

 Come back later, we're
 still working on this one...

Wikipedia :   Photoelectric effect vs. Geometrical optics.


(2015-05-08)   Minimal signal-to-noise ratio of a light sensor :
The ultimate limit depends only on the  number  of photons received.

This imposes a lower limit on the noise of the image sensors used on modern digital cameras.  Those are composed of a digital array consisting of millions of individual sensors of the type analyzed below:  One per pixel for a black-and-white sensor, up to four per pixel for color photography.

The arrival of photons in a monochromatic light beam is essentially a  Poisson process whose  activity  a  is equal to the  radiant power  of the beam  (in watts, W)  divided into the energy of each photon  (in joules, J).

For  standard  yellow-green light  (540 THz)  the luminous power in lumens  (lm)  is, by definition, 683 times the  radiant power  in watts (W). A surface area of  S  square meters receiving an illumination of  L  (expressed in lx, a  lux  being defined as a lumen per square meter)  thus receives an average number of photons per second equal to the activity in becquerels  (Bq)  of the aforementioned Poisson process, namely:

a   =   S (L / 683) / (h  5.4 1014 Hz)   =   L S  4.092 1015

If we express  a  in  Bq,   L  in  lx  and  S  in square microns,  we have: 

a   =   4092  L S

In a Poisson process with an activity of  a becquerels, the probability of observing exactly n arrivals in t seconds is given by:

Pn  =  exp(-lt) (at) n / n!

The  average  number of arrivals is  at.  Let  N  be the RMS of the noise:

N 2  +  (at) 2   =    S n   Pn  n 2

For the right-hand-side summation, we use the following remarks:

S n   x n / n!   =   exp (x)
S n   n  x n / n!   =   x d/dx exp (x)   =   x  exp (x)
S n n 2  x n / n!   =   x d/dx [ x exp (x) ]   =   x  exp (x)  +  x exp(x)

Applying this to the above with  x = at   yields:  N2 + (at) 2  =  (at) + (at) 2
So, the RMS value of the noise is  N = Ö(at).  and the signal to noise ratio is:

SNR   =   at / N   =   (at)½

 Come back later, we're
 still working on this one...

Wikipedia :   Image noise   |   Shot noise


(2015-08-23)   Counting photons without destroying them:
The work for which  Serge Haroche  was awarded a Nobel Prize (2012).

All ordinary light sensors are  receptors  of photons, which is to say that they absorb every photon they detect, thereby destroying it.

What Haroche discovered at the

 Come back later, we're
 still working on this one...

Wikipedia :   Serge Haroche (b. 1944)

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