Why do pictures look grainy when taken in low light?

Check your lens.

It’s because your automatic ISO setting puts it at a high ISO, which increases the gain to each pixel. This causes it to look grainy. You can minimize it by manually decreasing the ISO and decreasing the shutter speed, but your subject will need to be still.

The grain is visual “noise”. Try recording very quiet sounds with a tape recorder or something and play it back with the volume level turned up. You will hear more noise than if you recorded loud sounds and turned the volume level down. Cameras adjust the “volume level” (brightness or ISO level) automatically, so pictures are brighter in the dark.

Because the camera is creating “light” that isnt really there.

@deni Basically. The camera’s CCD or CMOS “film sensor” is going off accidentally all the time due to RF interference, but the level is generally low. When pictures in daylight are taken, the noise becomes insignificant in context with the bright pixels but are visible in low light when the brightness is automatically raised.

Interesting that this is also true for film cameras, where chemistry is at work in instead of electronics. High-speed film (for low light conditions) has bigger light-sensitive grains.

A film grain only registers light above a certain threshold. With less light coming in, you need a bigger grain to catch enough light to hit the threshold.