Since any opaque object absorbs the light, produces a shadow in space to its rear. If the light source is a point, the opaque surface eliminates all the light that reaches, producing a shadow of uniform density. One example is the shadow of one hand on a wall.
If the light source is grater than a point, the shadow varies in intensity, creating the umbra and penumbra. The first is that part where all the light rays are diminished, while the penumbra is the less dark part, not at all hidden to observers.
They are points of light: spotlights, torches, etc. The shadow most often found is the second type.
The kagashi-in-Jitsu rule is that the eyes first see the move, then the vulture and color in last place. The adjustment to the dark occurs when eyes will get used to the levels of illumination. It takes about thirty minutes to allow the cells to produce visual purple that allows the eyes to distinguish objects with very little light. The "outside center" vision is a technique to focusing the attention on an object without looking directly to it. When an object is directly viewed, the image is formed in the tapered eye region, which is not sensitive at night. When the eye see the object at ten degrees above, below, right or left, the image is formed at elongated cells, rod.
The rummage is a method of using this outside center vision to observe an object or an area. During the night observation, the elongated cells visual purple whiten within five to ten seconds, and the image disappears. When it happens, we must slightly change the eyes position, so that other elongated cells are used. We move, then, eyes, with short intervals in the object direction, but without looking directly. It must have pause of several seconds at each point of view, due to our eyes are normally used where there is sufficient light to create well clear contours and bright colors. In the darkness, the objects are loose, the outline is indistinct, and color is little or none. To move in the darkness, we must believe in what we see. Only the practice allows this kind of thing.