Update demo51.md

book/demos/demo51/demo51.md

@@ -107,10 +107,10 @@ name: demo51_fig4
 The setup schematically.
 ```
 
-We focus on what happens at B. Initially, thermal equilibrium exists throughout the space: the total power radiated by B equals the total power B absorbs from the whole space. Now, something very cold is placed at A. Over time, this affects the entire space, influencing B. But we focus on the immediate effects on B following the change at A. 
+First, consider the situation without mirrors. We focus on what happens at B, where B can be any point in space. Initially, thermal equilibrium exists throughout the space: the total power radiated by B equals the total power B absorbs from the whole space. Now, something very cold is placed at A. Over time, this affects the entire space, which in turn will influence B. But we focus on the immediate effects on B following the change at A. 
 
-Think of A and B connected by a cylinder $\pi$ (gray in {numref}`Figure {number} <demo51_fig4>`). B cools down because the radiative power that A sends through $\pi$ to B is now less than before, meaning less than the radiative power B sends through $\pi$ to A. More precisely: B experiences a net negative electromagnetic flux from the cylinder, causing B’s temperature to decrease. Without the mirrors, this would apply to every point in the space, and the 'cold source' would warm up while the entire rest of the space cooled. Since the emitted power is proportional to the fourth power of the temperature (in Kelvin), this change is quite a strong effect.
-But point B is special because the radiation that A emits within the solid angle $\Omega$’ is received by B from an equally large solid angle $\Omega$. So the radiative power reaching B via $\Omega$ also decreases, while B initially emits the same amount into $\Omega$. This makes B’s temperature drop faster than the area around B. For B, it’s as if A and the entire left mirror are ice-cold.
+Think of A and B connected by a cylinder $\pi$ (gray in {numref}`Figure {number} <demo51_fig4>`). Every point in space cools down because the radiative power that A sends through $\pi$ to B is now less than before, when it was equal to the radiative power B sends through $\pi$ to A. Consequently B experiences a net negative electromagnetic flux from the cylinder, causing B’s temperature to decrease. Without the mirrors, this would apply to every point in the space, and the 'cold source' would warm up while the entire rest of the space cooled. Since the emitted power is proportional to the fourth power of the temperature (in Kelvin), this change is quite a strong effect.
+But with the mirrors placed as shown in the figure, with point B in one focal point and A in the other, B is different from the rest of the space. It is no longer only the gray tube but all of the radiation that A emits within the solid angle $\Omega$’ that is received by B from an equally large solid angle $\Omega$. So the radiative power reaching B via $\Omega$ also decreases, while B initially emits the same amount into $\Omega$. This makes B’s temperature drop faster than that of all other points in space, in the area around B. For B, it’s as if A *and* the entire left mirror are ice-cold.
 
 ```{tip}
 As seen in {numref}`Figure {number} <demo51_fig1>` and {numref}`Figure {number} <demo51_fig2>`, we performed the experiment with two aluminum reflectors from old sun lamps, whose shape is far from parabolic. These also lack a clear focal point. We expect a more significant temperature drop with actual parabolic mirrors.
@@ -164,4 +164,4 @@ For those without parabolic mirrors (or something similar), you could try bendin
 ## References
 ```{bibliography}
 :filter: docname in docnames
-```
+```