A 2d cone is called a Light Cone.

<edit: Isn't it called a circle sector?>

a spreading cone of light

the world line of an object cannot leave its own forward light cone because to do so it would have to travel faster than light. once another object enters the forward light cone of an object, it cannot leave it either for the same reason.

The backward light cone is a different matter, but it is in the past. Objects can easily leave the backward light cone and no longer be visible.

Cones are the light sensors in your eye that sense the different wavelengths of colors. There are three different type of cone cell. An L cone, M cone, and an S cone.

the world line of an object cannot leave its own forward light cone because to do so it would have to travel faster than light. once another object enters the forward light cone of an object, it cannot leave it either for the same reason.

The backward light cone is a different matter, but it is in the past. Objects can easily leave the backward light cone and no longer be visible.