Reading Quiz 15 Key

Reading Quiz 15 Key

Due before class Friday, Jul 27th

Physics 106, Summer 2012

Reading: Chapter 25 Sec. 3

Did you complete the reading assignment?


Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links?


1) The optical properties of thin films are a result of interference of the reflection of light off the front of the film and the back of the film.

2) A 180 phase change is a result of a light wave reflecting off a medium that has a higher index of refraction than the original medium.

3) To observe thin film optical properties, the film must be near the same thickness as the wavelength of visible light.

Thin film optics can be very difficult to understand conceptually. The largest difficulties seem to arise in understanding what a 180 phase shift is and what causes it. The other difficulty usually arises in understanding how the interference in reflection from thin films causes a change in color.

Exercise 1

To gain a better understanding of what a 180 phase shift is go to: and read the first section (it refers to a 180 phase shift as a 1/2λ phase difference).

What is a 180 phase shift?

What condition will give a reflection with a 180 phase shift (select all that apply)?

Reflects off a rigid medium
Reflects off a non-rigid medium
Reflects off a less dense medium
Reflects off a more dense medium

To see how this applies to optics and thin films, go to:

The only buttons you need to worry about on this simulation is one from the 'n1 < n2 and n2 > n3' group and one from the 'n1 < n2 < n3' group.

View an animation from one wavelength of each group. Does a 180 phase shift occur when the reflection is off of a medium with a higher index of refraction than the original medium or a lower index of refraction than the original medium?

Higher than original
Lower than original

If you have a bubble and light hits the bubble from the outside you will have a reflection at two barriers. The first barrier from air to water (what the bubble's film is made of) and the second barrier from water to air (the inside of the bubble). Which reflection(s) will experience a 180 phase change?

First barrier
Second barrier
Neither barrier
Both barriers

Exercise 2

There are no available simulations that help with understanding why different colors appear as a result of thin films optical properties. Most places do not even try to address the issue either. If you go back to: and read the 'Oil and Soap Films' and 'Color Addition and Subtraction' sections it may help you understand this phenomenon (it is the simplest explanation I could find that really attempts to address the issue).

Notice that you can receive instant feedback to the questions asked at the end of the 'Color Addition and Subtraction' section which covers all the topics we have mentioned in this reading quiz. Rather than making up similar questions to what is already there and having you respond on this reading quiz. Just try to answer the questions on the website and check your answers there. If you have any questions about them, ask them in the first feedback box and Professor Magleby will be able to answer the questions during lecture.


The comments entered in these last two boxes go into a big anonymous data file that I will use to guide the lectures of the day. That means two things:

1) If you have a specific question or concern and would like an individual answer you will need to come by the office or send an email.
2) If you really want to get an anonymous comment to me before the lecture, it must be submitted before 11am that day.

Was there anything that you didn't understand in the reading assignment?  What was confusing to you?

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