What is shadow? What is penumbra?
They are evidence of the straight propagation of light and are explained in a simple and objective way by geometric optics, which is a branch of physics that studies light phenomena and their applications.
The principle of straight light propagation says that in a homogeneous and transparent medium, such as air, light propagates in a straight line. One of the consequences or applications of this light behavior is the formation of shadow and penumbra, which occurs when light encounters an opaque object in its path (any object that does not allow light to propagate through itself).
What will determine the formation of shadow or penumbra will be the type of light source, which can be a punctual or extensive source, let's distinguish the two cases:
1º: point source (light source with small dimensions in relation to what it will illuminate):
At figure 1, we have a point source F and an opaque sphere E.
A point source F emits light in all directions. The opaque sphere E does not allow light to propagate and thus the light rays do not reach the region behind the sphere. This unlit region is called
shadow.
Note that source F is small compared to the dimensions of sphere E, so we say it is a point or point source.
Looking at the figure, it can be seen that shadow it is a region where there is no light and occurs when the light source is punctual.
2nd extensive font (light source with considerable dimensions in relation to what it will light):
Now consider the figure 2 a light source F extensive.
With the light source extended, it can be seen in the figure that there is a region behind the opaque object that receives a small intensity of light from the source, not being totally dark, this is called dimness.
Soon, dimness it is a partially lit region and occurs when you have an extensive light source. Note that the light source and sphere have similar dimensions and are about the same size. Therefore, the source is said to be extensive.
A phenomenon that can be observed from Earth that originates through the formation of shadow and penumbra is the eclipse. O eclipse is a phenomenon that involves the Sun, Moon and Earth, and it can manifest itself in two ways, solar eclipse and lunar eclipse.
Solar eclipse
In the solar eclipse, the shadow and penumbra of the Moon are projected onto the Earth's surface and this will give rise to the eclipse, which can be total or partial. The eclipse will be total for observers who are in the shadow region and partial for observers who are in the twilight region.
Lunar eclipse
Sunlight when it touches the Earth causes a shadow to form on the back of the Earth, called the Earth's shadow. When the moon enters this region, observers on Earth can no longer see it, so we say that a lunar eclipse has occurred.
By Nathan Augusto
Graduated in Physics
Source: Brazil School - https://brasilescola.uol.com.br/fisica/sombra-penumbra.htm
