update practice exams

diff --git a/physics/light-matter-ref.md b/physics/light-matter-ref.md
index 4d8debb621d1dbeb9698160e8e90a2294127002c..8e3435663aa4b3d18c4eb7b38bf9c524a2a97aff 100644 (file)
--- a/physics/light-matter-ref.md
+++ b/physics/light-matter-ref.md
@@ -7,7 +7,6 @@ linestretch: 0.9
 ---
 
 \pagenumbering{gobble}
-<!-- \usepackage{multicols} -->
 
 # Light and Matter
 
@@ -78,7 +77,7 @@ $$\rho = {hf \over c} = {h \over \lambda} = mv, \quad E = \rho c$$
 
 <!-- ![](graphics/energy-levels.png) -->
 - $\Delta E = hf = {hc \over \lambda}$ between ground / excited state
-- $E$ and $f$ of photon: $E_2 - E_1 = hf = hc$
+- $E$ and $f$ of photon: $E_2 - E_1 = hf = {hc \over \lambda}$
 - Ionisation energy - min $E$ required to remove e-
 - EMR is absorbed/emitted when $E_{\operatorname{K-in}}=\Delta E_{\operatorname{shells}}$ (i.e. $\lambda = {hc \over \Delta E_{\operatorname{shells}}}$)