+
+![](graphics/double-slit-interference.png)
+
+Path difference $pd = |S_1P-S_2P|$ for point $p$ on screen
+
+Constructive interference when $pd = n\lambda$ where $n \in [0, 1, 2, ...]$
+Destructive interference when $pd = (n-{1 \over 2})\lambda$ where $n \in [1, 2, 3, ...]$
+
+Fringe separation:
+$$\Delta x = {{\lambda l }\over d}$$
+
+where
+$\Delta x$ is distance between fringes
+$l$ is distance from slits to screen
+$d$ is separation between sluts ($=S_1-S_2$)
+
+## Electromagnetic waves
+
+![](graphics/em-waves.png)
+
+- electric waves and magnetic waves are perpendicular to each other due to Faraday's law
+
+Wave equation:
+
+$$c = f \lambda$$
+
+where
+$c$ is velocity (speed of light in this case)
+$f$ is frequency (Hz)
+$\lambda$ is wavelength (m)