What is the noise in photographic works and why does it appear?

It mainly refers to the rough part of the image produced when CCD(CMOS) receives and outputs light as the received signal, and also refers to foreign pixels that should not appear in the image, usually caused by electronic interference. It looks like the image has been dyed and covered with some tiny rough spots. The digital photos we usually take may not be noticed if we use a personal computer to reduce the high-quality images and then look at them. However, if the original image is enlarged, there will be a color (pseudo-color) that did not exist originally, and this pseudo-color is image noise. -Baidu Encyclopedia

One more picture.

Those blue, red, blue and red ones in it are all noise. Welcome questions.