In fact, there is no special reason, that is, I can't sleep, simply go to see the western world, and it is already dawn.

Standing on the balcony, the rising sun slowly rises along the building on the right, pulling out a long shadow. What makes me curious is that the shadow is black and white at the beginning of the building. The farther away from the building, the dividing line between shadow and sunlight is less and less noticeable (or fuzzy).

*In my mind, I am thinking, why is this? Why is it not a straight one, black and white, but more and more blurred? *

I am an acute person (no memory), and immediately checked, the answer is very simple.

Penumbra and diffraction caused by:

When the light source is not a point source, the shadow formed by the illuminated object has umbra and penumbra. Penumbra is the shadow that we usually see on the edges. Although any obstacle can make the light diffract, the obvious diffraction is observed, and the conditions are still very harsh. So, strictly speaking, in general life, there are both penumbra and light diffraction at the edge of the shadow, but the blur that we can observe is mainly a penumbra.

A light source can be thought of as having many point sources. In the penumbra area, only a part of the point source can be illuminated, and the other part is blocked, so that a semi-dark part is formed.

From the penumbra, the closer to the umbra, the less the point source that can be illuminated, the darker it is. If you plan on the map, choose a point from the penumbra of the lower half to make the tangent of the earth and extend it to the sun.

It is very easy to understand. The sun below the extension line shines at this point, and the sun above it will not be visible. If you stand in the half of the earth, you will see a half-drawn sun. The partial eclipse is the earth in the penumbra of the moon.

May the glory of the First Charter shine through the universe