Beam diameter

Imagine you have a big flashlight that shoots out a beam of light. The beam is like the light that comes out of the flashlight, but it is in the shape of a circle. The size of the circle is called the beam diameter.

Now, if you shine the flashlight on a wall, the size of the circle of light that hits the wall is determined by the distance between the flashlight and the wall. If the flashlight is far away from the wall, the circle of light will be bigger. If it is closer, the circle of light will be smaller.

So, when we talk about beam diameter, we are really just talking about the size of the circle of light that is being emitted from the flashlight. And the size of the circle of light can be changed by moving the flashlight closer or farther away from the object it is shining on.