Lecture Notes on Thin Film

Illustration of Thin Film

• A thin film has a uniform thickness.
• Do not use a coloured picture in the exam.

Incident Light and Waves

• Incident light needs to strike at point B.
• Two waves emerge from point B:
  • Reflected wave (Yellow)
  • Transmitted wave (Green)
• Both reflected and transmitted waves have angle I.

Thickness and Amplitude of Waves

• Amplitude of the wave decreases with reflection, but draw all waves with the same thickness in the exam.

Interference Condition of Waves

• Reflected and transmitted waves show interference conditions.
• Measurements of angles and properties of the triangle are used.

Calculation of Path Difference

1. Calculate path difference ( \Delta x ).
2. ( \Delta x = 2 \times BC \times \mu - BG )
   • Here ( \mu ) is the refractive index.

Features of Angles and Triangle

• Study triangle BGD.
• Observe the alternate angles of reflection.
• Use properties of the triangle to calculate path difference.

Constructive and Interference Conditions

• For constructive interference: [ 2 \times T \times \mu \times \cos R + \frac{\lambda}{2} = (2n - 1) \frac{\lambda}{2} ]
• For destructive interference: [ 2 \times T \times \mu \times \cos R = n \lambda ]

Difference in Transmitted and Reflected Waves

• There is no phase shift for transmitted waves.
• There is a phase shift of ( \pi ) in reflected waves.

Conclusion

• Interference conditions of reflected and transmitted waves vary.
• Due to reflection, the intensity of reflected waves is higher.