See also: [[Umbra]] # Penumbra Motivation: Understanding [[Magnification for Radiography]] A penumbra, also known as an edge gradient or zone of geometric unsharpness, is defined as the region of partial illumination that surrounds the [[Umbra]]. It comes as a result of the fact that the focal spot isn't actually a point source, but rather has some small finite width. ![[Pasted image 20220907112620.png]] [^1] [^1]: [[@curryChristensenIntroductionPhysics1986]] Note that the region of penumbra can be of different sizes depending on the location of the source and object. This can be taken advantage of by putting the object of interest closer to the anode than the cathode. We can calculate the width of the penumbra as shown using similar triangles: ![[Pasted image 20220907114455.png]] [^1] This can be used in topics relevant to astronomy, when discussing solar eclipses, in which the [[Umbra]] is the area of the sun that is covered up, and the penumbra is the zone between the region of full shadow and full light.