shadow and twilight
Shadow and penumbra arise when a screen is placed in front of an extended source of light.
To understand the difference between shadow and penumbra , we need to understand two important issues concerning geometric optics :
I. The so-called principle of rectilinear propagation of light says that, in homogeneous media, light always propagates in a straight line;
II. Light sources that have negligible dimensions are called point sources. Those that have considerable size, such as the Sun, are called extensive sources.
The image below shows that when an opaque screen is facing a point source of light, it will receive a portion of the light rays, but the part opposite the point source will be dark. This region that was not hit by light rays is called shadow , which, by definition, is understood as a region where there is no light.
Definition of Penumbra
The Sun, because it has its own light, is considered a primary source of light and, as it has very large dimensions, it is called an extended source. When the Moon comes between the Earth and the Sun, with the three stars remaining aligned, we have a situation similar to the one shown in the figure above, as the shadow and penumbra of the Moon are projected onto the Earth, causing the so-called solar eclipse . In shadow regions, we say that the eclipse was total and the Moon completely covers the Sun. In the penumbra regions, the eclipse is called partial and the Moon covers only part of the Sun.