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diff --git a/docs/lecture_1.md b/docs/lecture_1.md
index 471fd8403f3f591d03a954e6f248111e6178fcf6..0d6d176702977acef6f0ba0ec432118269217db4 100644
--- a/docs/lecture_1.md
+++ b/docs/lecture_1.md
@@ -49,7 +49,7 @@ This is for just one atom. In order to obtain the heat capacity of a full materi
### Debye model
The Einstein model explained the experimental data quite well, but still slightly underestimated the observed values of $C$. Apparently the "each atom is an oscillator"-idea is too simplistic.
-
+
Peter Debye (1884 – 1966) suggested to instead consider _normal modes_: sound waves that propagate through a solid with velocity $v=\omega/k$, where $k=2\pi/\lambda$ is the _wave number_. Instead of just multiplying $\bar{\varepsilon}$ by $3N$, we now use: