It’s usually only with the cheaper led bulbs. I can’t convince others that I’m only seeing it flicker when not directly looking at it.
It has to do with the structure of the human eye and the nature of peripheral vision.
The light is flickering all the time, even when you’re looking straight at it, but peripheral vision is much better at detecting flicker and motion than the focal point of your eye. That’s why you can notice the flicker when it’s off to the side, but the light appears to smooth out as you focus on it.
Streaks of light that appear briefly in your visual field are known as flashers. Some people compare these flashes to lightning or shooting stars.
Eye flashers are associated with the vitreous humor inside your eye. Vitreous humor is a gel-like substance that fills the majority of your eyeball. This gel allows light to enter the eye via the lens, and it is connected to the retina. If vitreous gel bumps or pulls on the retina, you may see flashes of light in the corner of your eye.
TL;DR: Just the eye’s rod cells in your peripheral vision pick up the flickering of lights. The lights flicker due to the power-grid using alternating current, reversing which way power flows 60 times a second.
The power grid functions with alternating current (AC). Like if you would rotate a battery around, just very fast (60 times a second).
This is done as you can transfer power over power-lines more efficiently that way (at least as it’s done at the moment).
It’s 60Hz in the US, and 50Hz in other parts of the world.
More modern LED lights may not show this flickering due to full bridge rectifiers and capacitors, changing it so
current flows in one direction only. Only some cheap, cheap LED lights just hook up to the power directly with some resistor and basically turn off half the time (as LED will only turn on if current flows in one certain direction – DC = Direct Current).
But old incandescent light-bulbs are simpler. Just a wire that heats up. And power running directly back and forth.
It’s most noticeable in an old TV.
Your eye / retina has two different type of cells:
Cone cells. They perceive color and are most dense in the center of your vision. (And there are three different types of those for red, green and blue light.)
Rod cells: They dominate your peripheral vision, and can not perceive color. But they are more sensitive. E.g. when you’re outside at night, and seeing stuff at all is more important than seeing things in color.
And they also pick up the flickering frequency, while the cone cells do not. So slow!