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diff --git a/2020/04/hyperphysics/light_and_vision/index_of_refaction.md b/2020/04/hyperphysics/light_and_vision/index_of_refaction.md
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+
+; http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr.html
+
+(eq nu (/ c v))
+
+(var nu "index of refraction")
+(var c "velocity of light in vacuum")
+(var v "velocity of light in the medium")
+
+; Material examples for nu:
+;    Vacuum  1.000
+;    Air     1.000277
+;    Water   4/3
+;    Ice     1.31
+;    Crown glass 1.50 to 1.62
diff --git a/2020/04/hyperphysics/light_and_vision/inverse_square_law.md b/2020/04/hyperphysics/light_and_vision/inverse_square_law.md
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+
+; see power_solid_angle
+
+(eq E
+    (/ I (exp r 2)))
+
diff --git a/2020/04/hyperphysics/light_and_vision/power_solid_angle.md b/2020/04/hyperphysics/light_and_vision/power_solid_angle.md
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+
+; "lux" is lumens / m^2
+
+; (var 'symbol 'name 'units)
+
+(var E "illuminance" 'lux)
+(var Theta "luminous power" 'lumens)
+(var I "pointance" 'candela)
+(var Omega "solid angle" 'sterradians)
+
+(eq Theta
+    (* 4 pi I))
+
+(eq E
+    (/ Theta A)
+    (/ (* I Omega) A)
+    (/ (* I A) (* A (exp r 2)))
+    (/ I (exp r 2)))
+
+(eq Omega
+    (/ A (exp r 2)))
+
+(eq I
+    (partial-derivative Theta omega))
diff --git a/2020/04/hyperphysics/light_and_vision/raman_scattering.md b/2020/04/hyperphysics/light_and_vision/raman_scattering.md
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+
+(var DeltaE "change in energy")
+
+(eq DeltaE
+    (* (/ hbar (* 2 pi I))
+       (+ (* 2 J)
+          3)))
diff --git a/2020/04/hyperphysics/light_and_vision/rayleigh_scattering.md b/2020/04/hyperphysics/light_and_vision/rayleigh_scattering.md
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+
+; http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c1
+
+(var N "number of scatterers")
+(var alpha "polarizability")
+(var R "distance from scatterer")
+(var I_0 "original intensity")
+(var I "scattered intensity")
+(var lambda "wavelength of light")
+(var Theta "angle of scattering")
+
+(eq I
+    (* I_0
+        (/ (* 8 (exp pi 4) N (exp alpha 2))
+           (* (exp lambda 4) (exp R 2)))
+        (+ 1 ((exp cos 2) Theta))))
+
diff --git a/2020/04/hyperphysics/light_and_vision/snells_law.md b/2020/04/hyperphysics/light_and_vision/snells_law.md
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+
+Snell's Law
+
+(eq (/ n_1 n_2)
+    (/ (sin Omega_2)
+       (sin Omega_1)))
+
+(Omega_1 "angle from boundry for first medium")
+(Omega_2 "angle from boundry for second medium")
+(n_1 "index of refaction of first medium")
+(n_2 "index of refaction of second medium")
+
+
+good example is the 22 degree halo: Ice (n=1.31), two refractions from 60
+degree prism (hexagonal ice crystal)
diff --git a/2020/04/hyperphysics/values.md b/2020/04/hyperphysics/values.md
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+
+; (contant 'symbol 'name 'units 'value)
+
+(constant c "velocity of light in vacuum" (/ 'meters 'second) 2.99792458e10)
diff --git a/2020/04/latex.md b/2020/04/latex.md
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+
+KaTeX table of supported/not-supported characters: <https://katex.org/docs/support_table.html>
diff --git a/2020/04/mathml.md b/2020/04/mathml.md
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+
+<https://www.w3.org/TR/MathML3/>
+
+<https://en.wikipedia.org/wiki/MathML>
+
+"Strict Content MathML" is a subset, compatible with Open Math (which is itself an XML encoding).
+
+Systems Biology Markup Language (SBML) is another XML schema for representing
+models of biological systems and chemical reactions.
+=> <http://sbml.org/More_Detailed_Summary_of_SBML>
+
+There are a bunch of non-printing "entities" (characters) with Unicode mappings:
+
+U+2061 FUNCTION APPLICATION;
+U+2062 INVISIBLE TIMES;
+U+2063 INVISIBLE SEPARATOR;
+U+2064 INVISIBLE PLUS;
+
+
+Presumably, we'll want to support reading *content* MathML, and output eithr
+content or presentation.