Lecture Notes: Thin Film Interference

Key Concepts

Wavelength in Different Materials
- Light has a specific wavelength in each medium it travels through.
- Wavelength in air (Material A) is different from wavelength in oil (Material B).
- Wavelength in the thin film is critical to calculate because it determines constructive or destructive interference.
Interference Conditions
- Light traveling extra distance in the film affects interference.
- Constructive interference occurs when waves emerge in phase.
- Destructive interference occurs when waves emerge 180 degrees out of phase.

Frequency Consistency
- Frequency remains constant across different media.
- Determined by the source of light (e.g., sun, laser).
- Formula relationship: Speed = Wavelength x Frequency.
Calculating Wavelength
- Use speed to find wavelength in thin film:
  - Wavelength in B = (Speed in B / Speed in A) * Wavelength in A
- Index of refraction is another method:
  - Wavelength in B = (NA / NB) * Wavelength in A

Index of Refraction Definitions
- NA = Speed of light in a vacuum / Speed in A
- NB = Speed of light in a vacuum / Speed in B
- Calculate speeds using:
  - VA = C / NA
  - VB = C / NB

Constructive Interference Condition
- 2t = m * Wavelength in B where m = 0, 1, 2...
- Simplified using velocity and index of refraction:
  - m * (Velocity or Index ratio) * Wavelength in A
Destructive Interference Condition
- 2t = (m + 0.5) * Wavelength in B
- Similar simplification as constructive condition.

Pi Shift
- Pi shift can flip constructive and destructive conditions.
- Only one wave getting a pi shift causes this change.
Air Medium Simplification
- Index of refraction in air simplifies calculations (NA = 1).
- Leads to easier formula for interference conditions.

Thin Film Uses
- Anti-reflective coatings are very thin films.
- Thin films can cause destructive interference, reducing reflection.