Using The Wavelength of light (λ) as a Ruler to Measure the Thickness of a Thin Film

 

Audience

10th, 11th, 12th grade science

Time Frame: 60 min period or less depending on how long it takes for the required content before the activity

Set-up:

Activity 1: (5 min)

Objective(s)

After completing the activity, participants will be able to:

  1. Explain that X-Ray reflectometry is a tool used to measure the thickness of a thin film.

  2. Identify applications where thin films play an important role.

  3. Apply mathematical formulas when to calculate experimentally the thickness of a thin film made of nail polish using the wavelength of light as a ruler.

  4. Understand that our vision and physical tools have limitations

 

 

Standards Addressed

Next Generation Science Standards PS 4

1. Construct explanations for why the λ of an electromagnetic wave determines its use for

certain applications

2. Make inferences and justify conclusions from sample experiments

3. Analyze that multiple technologies are based on the understanding of waves and their

interactions with matter are everyday experiences in the modern world

4. Reason quantitatively and use units to solve problems

National Science Education Content Standards

  • Students relate and apply concepts of energy and its interaction with matter to recognize types of waves and explain how they transfer energy, and to explain how wavelengths can be used to identify samples of matter.

  • Students manipulate, communicate and present data using appropriate statistics, mathematical models, and available technology

.

Engineering principles

Grade Band End Points for ETS2.B

By the end of grade 12. Modern civilization depends on major technological systems,

including those related to agriculture, health, water, energy, transportation,

manufacturing, construction, and communications. Engineers continuously modify

these technological systems by applying scientific knowledge and engineering

design practices to increase benefits while decreasing costs and risks. Widespread

adoption of technological innovations often depends on market forces or other

societal demands, but it may also be subject to evaluation by scientists and engineers and

to eventual government regulation. New technologies can have deep

impacts on society and the environment, including some that were not anticipated

or that may build up over time to a level that requires attention or mitigation.

Analysis of costs, environmental impacts, and risks, as well as of expected benefits, is a

critical aspect of decisions about technology use.

 

Activity Materials

Part 1(Groups of two) Each group will need the following:

A small round bowl with a flat surface or glass/plastic Petri dishes. A piece of thin copper wire around 9 inches (23 cm).Water to cover the wand in the bowl or Petri dish and ONE drop of clear nail polish.

Part 2(Groups of two) A six by six in. piece of black poster board, a flat plate or pan to place poster board, water to immerse the poster board, ONE drop of transparent nail polish, one red LED flashlight and one green LED flashlight, scissors, two colored pencils or pen, tape and white paper.

 

Activity Instructions

Set-up: 10 minutes setting up and conducting part 1

Part 1- The first activity is part of the lesson plan. Students will make a thin film and try to fish it out and observe any color interference that would be an indication of a thin film. Through open discussion, students realize that you cannot get too much information about the thin film if it does not have a substrate to place the thin film on it.

 

Procedure for Part 1

Introduction (see background information) 30 minutes

Part 2 of the Activity (10 min)

 

Place black paper in a tray and cover it with the smallest amount of water.

 

Make sure the paper is completely under water.

Add one drop of nail polish and wait approximately 1 minute to remove it slowly by the ends.

( Do not let the film slip from the paper.)

Let it dry

NOTE: In order to save time, the teacher can give out other thin film prints already made that are dry so the quantitative part of the activity can continue.

The results should look like this:

 

 

 

 

 

The discussion should lead to the fact that since white light has all the wavelengths, white light is not a good "ruler" to measure how thick the thin film is.

Now, the film will be exposed to specific lights that have specific wavelengths. Start with the red and count the DARK fringes that you see. Record this number. The dark fringe is where destructive interference occurs. Repeat the process with the green light. If done carefully, the students will notice that for every 4 red fringes, there will be 5 fringes with the use of the green light. This is why it is important to make the thin film printout to be used, with many rings. If you have less than 4 fringes of red, the green will also be the same amount. The ideal situation to observe the trend is being able to make thin films that you can see eight fringes with the red light and ten with the green light. This can be the students’ independent research; to make a thin film that shows this effect. The reason for this difference is that green light has a shorter wavelength than red so this "ruler"can be more accurate. Make sure that the same section is observed so proper comparisons can be made.

 

 

The introduction to the table below should be discussed with the students:

Where do the dark fringes come from?

Light reflects from surface and from the bottom.

In order for the waves to be out of phase, there is a difference in path of λ/2. If the waves are in phase, constructive interference occurs. The dark fringes are a result of destructive interference.

The third part of the activity (5 min)

This activity will show that there is a difference in the view of the thin film fringes under two different colors of light.

 

 

 

Instructions: See diagram below

 

 

 

 

The following picture shows the same thin film under the different lights. Notice that the fringes are not exactly seen at the same position. The reason for this is that the wavelength of green light is shorter that the wavelength of red.

 

 

 

 

 

 

 

 

 

The Fourth Part of the Activity (10 min)

This is the part where the students will calculate the thickness of the film using the data collected when counting the dark fringes with the red light and the green light.

Thickness (t) = # dark fringes ( λ/2)

Example: red : counted 4 fringes. λ for red light through plastic is 458 nm

t= 4 x λ/2

t= (4) 458/ 2 = 916 nm with red light

 

Example: green: counted 5 fringes. The λ for green light through plastic is 366 nm

t= (5) 366/2 = 915 nm

This is just an example if you actually see these numbers of fringes. Remember that a student can get the same amount of fringes if the proportion is not 4 to 5 as explained earlier. This is where analyzing the results of all the group is of great benefit if a group sees only 3.

Conclusion ( 5 min)

In conclusion, we have a ruler as accurate as λ/2

Discussion on how could we be more accurate? By using a "ruler" with a very short wavelength such as X-Ray Reflectometry. X-Rays have a wavelength of 1.5 Å that is equivalent to 0.15 nm; a thousand times better ruler!

Assessment

1. Name four applications for thin films

2. Why are thin films necessary? What are the limitations of our vision in determining thickness?

3. What can be a better ruler to measure the thickness of a thin layer and why?

4. What would we need to know in order to determine the validity of our data?

5. What can you conclude when analyzing the calculations made using two different lights?

6. Which light was a better ruler? Justify your answer

7. Provide an educated explanation in terms of what you think will happen if we use an LED

blue flashlight for this activity. Design an experiment to test your hypothesis.

 

Background

There should be an introduction about the technology known as thin films. The major points about this topic are the following:

  • What are thin films?

  • Examples of common thin films

  • Applications of thin films

  • Methods to build thin films

  • How is the thickness of thin films measured and why is it important to know it?

Concepts that need to be developed if not known yet:

    • The relation between wavelength and color

    • Visible light wavelength range

    • Thin film interference

    • Constructive and destructive interference

    • Index of refraction

    • Phase change and λ / 2

    • Basic law of reflectivity

The following diagram helps in explaining when you see fringes in a thin film

When ray 1 hits the surface on the top, some of the light is partially reflected as seen in ray2 and the rest is refracted as seen in ray 3. When ray 3 hits the bottom surface, some of it is reflected as seen in ray 4 and the rest is refracted as seen in ray 6.

When ray 4 hits the top surface from underneath some is reflected and some is refracted as seen in ray 5

Figuring out the path length difference that the surface reflected ray takes and the internal reflected ray can determine the thickness of the film that will cause a particular λ of light to be reflected. References

Interference: Thin film interference and reflection. (n.d.) In Physclips. Retrieved July 10, 2013, from http://www.animations.physics.unsw.edu.au/light/interference/   

Physical optics Thin film Interference. (n.d.) In Physical Optics. Retrieved July 10, 2013, from http://dev.physicslab.org/Document.aspx?doctype=3&filename=PhysicalOptics_ThinFilmInterference.xml   

Thin Film Interference. (n.d.). In Physiscs Classroom. Retrieved July 10, 2013, from http://www.physicsclassroom.com/Class/light/u12l1c.cfm   

What Wavelength Goes with Color? (n.d.). In National Aeronautics and Space Administration. Retrieved July 10, 2013, from http://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html

 

 

Authors

Evelyn Montalvo, RET teachers

MRSEC IEG Leadership Team: Ben Taylor, [ Paul Evans, Margaret Cosgriff, and Anne Lynn Gillian-Daniel.