Trigger an Event when some element appears on the screen, (like hes scrolling when appears the element trigger something) , how to?

Trigger an Event when some element appears on the screen, (like hes scrolling when appears the element trigger something) , how to?

Trigger an Event when some element appears on the screen, (like the person is scrolling when the element that we set appears, the event trigger) , how to do that, does anyone know?

thanks in advance. ^^

I also would like to know how Google Images does, at first it appears to look like a gigantic page, because the scroll is very big, but actually the content loads only when you position your scroll in the area. A very smart system if anyone knows how to do that, I would also be very pleased! ^^