Thin Coatings

Section 18.10 Thin Coatings

Subsubsection Activities

Activity 18.10.1. Warm-up.

Recall the Thin-film Figure, in which light is incident on a thin surface with a different index of refraction than the neighboring surfaces.

🔗

(a)

A common assumption when considering thin-film interference is for the incident light to be nearly normal to the surface. How does this assumption simplify the situation?

🔗

(b)

How would you redraw the figure to represent the situation more accurately? Why do you think the figure is drawn the way it is?

🔗

(c)

What is the path length difference between rays 2 and 5 for the light traveling through the thin film to the right?

🔗

(d)

What is the phase difference due to the path length difference for the rays 2 and 5 traveling through the thin film to the right?

🔗

(e)

When light reflects off a material of a higher index of refraction, the reflected wave is inverted, which can be represented as a phase shift of \(\pi\text{.}\) How does this compare to other situations you have seen in which a reflected wave is inverted? Check your answere in Boundaries

🔗

Activity 18.10.2. The Screen Coating.

🔗

A thin layer of \(\mathrm{TiO_2}\) with \(n \approx 2.6\) is a common coating for the glass screen of an electronic device \(n \approx 1.5\text{.}\)

🔗

(a)

For each reflection, identify how (if at all) the phase of the reflected light will shift.

🔗

(b)

How thick should the coating be to cause complete destructive interference for light with a wavelength of 520 nm? (Hint: Use your expressions from the previous activity!)

🔗

(c)

After some testing, you see that the thin coating breaks down too fast when it is this thin. What are other thicknesses that would still produce the desired result?

🔗

Activity 18.10.3. Simulation.

🔗

Visit this simulation and use it to check how your answer would change if you adjusted the material out of which the thin coating was made.

🔗