Thin Films

17.4

Observations

In small groups, take turns blowing bubbles from your bubble mix. Notice the color of the bubbles and when it appears that particular color.

 Observe the Newton’s Rings as well.

How could you use the wave model of light as a way of explaining the phenomena?

How can interference be occurring in the thin film?

Visit the following website and view the Java Applet.

Set both the “light source” and “show interference pattern” to blue, and the indices of refraction to n1=1, n2=1.5, and n3=1. Then click play.

1. The incident wave travels downward in the simulation. Is the first reflected wave from the top layer of the thin film inverted or in phase? Is the second reflected wave from the bottom layer of the thin film inverted or in phase?

2. Carefully observe the light wave on the left. What happens to its wavelength as it moves through the three different material? What does the wavelength depend on?

3. Adjust the thickness to zero and then slowly increase the value until the two reflected waves interfere constructively. Record the first three thicknesses. Repeat for destructive interference.

4. Now change the index of refraction of the third material to 1.7. Is the first reflected wave from the top layer of the thin film inverted or in phase? Is the second reflected wave from the bottom layer of the thin film inverted or in phase? Record the first three thicknesses for constructive and destructive interference in the table on the next page.

Fill in this table summarizing your findingsthicknesses for

constructive interference

thicknesses for destructive interference

n1=1, n2=1.5, and n3=1

72, 216, 366 144,294, 425

n1=1, n2=1.5, and n3=1.7

constructive interference

destructive interference

when both reflections are

inverted or both in phase

2tn=mλ 2tn = (m-1/2)λ

when one reflection is

inverted and the other in phase

2tn = (m-1/2)λ 2tn=mλ

Example #1White light illuminates an oil film on water. Viewing it directly from above, it looks red. Suppose that the thickness of the film is 300 nm. What is the refractive index of the oil? Assume that the refractive index of water is greater than oil. Also take red light to have a wavelength of 700 nm.

Answer

Example #2

What is the minimum thickness of a soap bubble needed for constructive interference in reflected light, if the light incident on the film is 750 nm? Assume that the index for the film is n=1.33.

Answer