Electromagnetic WAVES arriving at a detector surface must be absorbed, if they are to be detected.
And at the time of absorption, the energy of the WAVES turns out to be quantized.
These QUANTA of electromagnetic waves are called PHOTONS.
Photons absorbed in the pixel areas ... perhaps a bit abstract.
Let us try to build an ALLEGORY: Raindrops collected in BUCKETs (1).
Imagine that you place a number of empty buckets outside in a gentle uniform rain.
One hour later, you measure the water collected in each bucket.
Mean contents is a measure of rain intensity.
Difference between biggest and smallest contents is measurement uncertainity (noise).
Now imagine that you replace the buckets by the same number of thimbles, accordingly reducing collection time.
While one thimble might remain dry, another one might overflow.
Clearly the smaller number of drops collected results in a much larger MEASUREMENT NOISE.
But we won't stay playing in the rain.
We need solid facts and numbers to describe noise in pictures.
So look what Poisson can do for us.
text ref. (1)
Dietmar A. Dowe: "Death, Taxes, and Shot Noise",Photonics Spectra,July 2000, page 122f.