Question

Why does image noise happen?

Answer 1

HEY MATE
Here is your answer
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• $\bf\color{blue}\mathcal{Image\:noise}$ is random variation of brightness or color information in images, and is usually an aspect of electronic noise.

• It can be produced by the sensor and circuitry of a scanner or digital camera.

• Image noise can also originate in film grain and in the unavoidable shot noise of an ideal photon detector. 
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Sources : www.wikipedia.com
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Hope this helps!!!!!

Answer 2

On smaller image sensors which cram more photosites into a smaller area, this effect can be magnified. Another common cause of noise is shooting at higher ISO settings. As these settings basically magnify the light signal, they also magnify other unwanted signals such as background interference (eg. heat sources).