**Basics**

- A
**wave**is a disturbance in a medium that carries energy. - Two points are
**in phase**if they have the same motion at all times - Two points are
**completely out of phase**if their motions are always opposite - Superwaves are created when two identical waves move toward each other

**Path Length and Path difference**

- Path length: the distance from a point to the source of a wave
- Path difference(Δ): the difference between the path lengths of two different waves
- If two point sources vibrate in phase with each other, they would be in-phase at points where Δ is a whole number multiple of λ

**Waves at Boundaries**

- The frequency of a wave never changes
- Snell's law: n×sin(θ) is constant (n is the index of refraction)
- If a wave travels from a faster medium to a slower medium, the transmitted wave becomes inverted

**Interference from two point-sources**

- If a point P is very far away from the two sources, then Δ= d×sin(θ), where d is the separation between the sources, and θ is the angle between
__the perpendicular bisector of the sources__and the line joining P and the midpoint of the two sources

**Young's double slit experiment:**

- Problem: to have two point sources of light that are coherent
- The first slit acted as a point source of light
- The double slit acted as two point sources of light
- On the screen, the bright fringes are where the antinodal lines are
- The dark fringes are where the nodal lines are

**Interference of thin films:**

- Use the theory of "waves at boundaries" and path difference to do these problems
- n
_{1}×λ_{1}=n_{2}×λ_{2}