update practice exams

diff --git a/physics/light-matter.md b/physics/light-matter.md
index a2d5f6dfc60580a251e2eee8b3ce6b07a33daedb..ce114bac8b0daeddb34a5c65ac2b632fb06d4adb 100644 (file)
--- a/physics/light-matter.md
+++ b/physics/light-matter.md
@@ -110,7 +110,8 @@ $$\rho = {hf \over c} = {h \over \lambda}$$
 
 
 ### Absorption
-- Black lines in spectrum show light not reflected  
+- Black lines in spectrum show light not reflected
+- Frequency of a photon emitted or absorbed can be calculated from energy difference: $E_2 – E_1 = hf$ or $= hc$
 
 ### Emission
 
@@ -171,6 +172,17 @@ Therefore, position and velocity cannot simultaneously be known with 100% certai
 
 **Quantum mechanical model** - electron clouds rather than discrete shells (electrons are not particlces). We can only calculate probability of an electron being observed at a particular position
 
+Newton's and Einsteins models work together  
+
+### Photon-electron interaction
+
+When a photon collides with an electron, momentum is transferred to electron.
+
+$$\rho_{\text{photon}}={h \over \lambda}$$
+$$E=\rho c$$
+
+
+
 
 
 774 abc melbourne
\ No newline at end of file