This is the teacher guide for this lesson. A student-focused guide to assist learners as they perform the activity is available.
Teacher Guide
Hair Thickness
This resource was originally published in PhysicsQuest 2009: Power!
How can a laser be used to measure the width of a human hair?
- Laser pointer
- Binder Clips (2)
- Index card
- Masking Tape
- Scissors
- Ruler
- Large sheet of paper
- Pencil
- Human hair (Ask nicely before plucking this from your friend's head)
Students start by discussing how thick their hair is, struggling with the concept of measuring such a small distance. This will get their curiosity sparked to engage in the experiment of using laser wavelengths to measure a piece of hair. They will collect and analyze data to answer the question, “How can a laser be used to measure the width of a human hair?”
- Total time45 - 60 Minutes
- Education levelGrades 5 - 9
- Content AreaWaves
- Educational topicWavelength, interference, diffraction
Let's start by thinking about what happens when waves interact. We will assume that we are dealing with two waves that have the same wavelength. When two waves come together they add up to make a resultant wave. If the two waves have crests and troughs at the same points, they add up to make a wave that is twice as big as the two individual waves. This is called "constructive interference." When two waves have crests and troughs at the same points they are said to be "in phase." If two waves are completely "out of phase" which means one wave is up while the other is down, they cancel each other out. This is called "destructive interference." When water waves flow around a pylon because they spread out on the other side of the pylon they will come back together and interact with each other. Sometimes they constructively interfere and sometimes they destructively interfere. If we were to measure the height of the waves on the other side of the pylon we would sometimes see big waves and sometimes see no waves at all. You have always heard that light always goes in a straight line. Well it does. Except when it doesn't. When light encounters an obstacle that is really small, only slightly larger than the wavelength of light, it will spread out again as it passes the obstacle, just like water flowing around a pylon. This is called diffraction. So light behaves just like the example of water flowing around the pylon that we talked about before. But what does this have to do with finding the thickness of a hair? Well, as the light goes around the hair it is going to come back together at different points based on the thickness of the hair and the wavelength of light. Sometimes the light waves will cross when they are both up, sometimes they will cross when one is up and one is down. It will depend on the wavelength of light and how far it has traveled. If the wavelength of light is smaller, say, blue or green light, then the pattern you would see would have the bright spots closer together. What do you think you would see if you could do this with white light? So far we know the pattern on the wall depends on at least two different things, the thickness of the hair and the wavelength of the laser. Does it depend on anything else? Imagine what would happen if you moved the hair closer to the wall. The pattern would change. In fact, the spots would get closer together. The mathematical way of saying all this is: thickness of hair=((wavelength of laser) * (Distance from hair to wall))/(average spacing between bright spots). So to find the thickness of the hair, we need to know the wavelength of the laser. If you have done activity one you can use the value of wavelength you found there. If not, or if you are not sure of your answer to activity one, the wavelength of your laser is 670nm. In doing this experiment it is going to be necessary to pay close attention to units of distance. I would recommend changing everything to meters and then changing micrometers to determine your final answer. This will allow students to get practice in unit conversion.
Key terms
These are the key terms that students should know by the END of the two lessons. They do not need to be front loaded. In fact, studies show that presenting key terms to students before the lesson may not be as effective as having students observe and witness the phenomenon the key terms illustrate beforehand and learn the formalized words afterwards. For this reason, we recommend allowing students to grapple with the experiments without knowing these words and then exposing them to the formalized definitions afterwards in the context of what they learned.
However, if these words are helpful for students on an IEP, ELL students, or anyone else that may need more support, please use at your discretion.
- Wavelength: The distance from one wave peak to the next.
- Diffraction: When light goes around an obstacle or through a single slit the light rays interact with each other. When they do, a pattern of dark and bright spots is created.
- Interference: When light passes through two slits and the light rays from each of the slits interact. It is like diffraction but involves more than one slit or obstacle.
- Constructive Interference: When two waves come together and make a bigger wave.
- Destructive Interference: When two waves come together and cancel each other out.
- In Phase: When two waves are going up and down together.
- Out-of-Phase: When one wave is going up as the other is going down.
Before the Experiment
Ask & Discuss
- How thick do you think your hair is?
How thick do you think your hair is?
- Use the Snowball discussion protocol
Use the Snowball discussion protocol
- Students begin in pairs, responding to a discussion question only with a single partner.
- After each person has had a chance to share their ideas, the pair joins another pair, creating a group of four.
- Pairs share their ideas with the pair they just joined.
- Groups of four join together to form groups of eight, and so on, until the whole class is joined up in one large discussion.
Setting Up
- Cut out the center of a 3 in. x 5 in. index card
Cut out the center of a 3 in. x 5 in. index card.
- Have everyone in the group donate a hair
Have everyone in the group donate a hair. Ask nicely as this may hurt a bit!
- Tape the hair in the middle of the index card
Tape the hair in the middle of the index card. (Fig. 1)
- Attach binder clips to sides of card so that it will stand up
Attach binder clips to sides of card so that it will stand up. (Fig. 2)
- Place the laser pointer on top of a book so that the laser beam will shine directly on the hair
Place the laser pointer on top of a book so that the laser beam will shine directly on the hair and tape it down securely. (Fig. 3)
- Put this set-up 4 meters from a wall and tape the large piece of paper to the wall
Put this set-up 4 meters from a wall and tape the large piece of paper to the wall so that the laser beam will shine on it.
During the Experiment
Collecting Data
- Make sure students are put into intentional groups
Make sure students are put into intentional groups.
- Students will complete the experiment using the Student Guide
Students will complete the experiment using the Student Guide where we have outlined the experiment for students and along the way, they record results and answer questions.
Analyzing Data
- Students will look at their data to describe the patterns they saw so they can draw conclusions about human hair
In the student guide, they will look at their data to describe the patterns they saw so they can draw conclusions about human hair.
- Listen in on each group’s discussion
Continue to listen in on each group’s discussion, answer as few questions as possible. Even if a group is off a little, they will have a chance to work out these stuck points later.
Teacher Tips
- Suggested STEP UP Everyday Actions
Suggested STEP UP Everyday Actions to incorporate into activity
- When pairing students, try to have male/female partners and invite female students to share their ideas first
- As you put students into groups, consider having female or minority students take the leadership role.
- Take note of female participation. If they seem to be taking direction and following along, elevate their voice by asking them a question about their experiment.
- Consider using white boards
Consider using white boards so students have time to work through their ideas and brainstorms before saying them out loud.
- Roam around the room to listen in on discussion and notice experiment techniques
As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.
- Consider using the RIP protocol
Consider using the RIP protocol (Research, Instruct, Plan) for lab group visits and conferring.
- Consider culturally responsive tools and strategies and/or open ended reflection questions
Consider culturally responsive tools and strategies and/or open ended reflection questions to help push student thinking, evidence tracking, and connections to their lives. Look for *** below to find suggested places to add
Conclusion
- have students use the Write-and-Pass protocol to clarify their thinking
Before students write their answers for #2 in the student guide, have them use the Write-and-Pass protocol to clarify their thinking.
- Divide students into groups of four (each from a different experimental group)
- The teacher posts a question that students must answer with an explanation.
- Students each write their own ideas on a loose piece of paper.
- Then the papers are all passed to the left.
- Each student silently reads the student’s response (and any of the other students’ comments, on iterating rounds of this process).
- Each student writes suggestions directly onto the original copy to help make their peers’ ideas sharper and clearer.
- Repeat the pass-read-edit until each student gets to read and comment on each others’ ideas.
- The original author of each statement reads their peers' comments and writes a refined, final statement in their student guides.
- Clarify and give concise definitions
After students have had a chance to discuss key ideas from the lesson, you can now clarify and give concise definitions to the forces they experimented with.
- Real world connections
- Search and discuss Rosalind Franklin’s use of diffraction to discover the shape of DNA. What else could you measure or discover using a laser?
- Suggestions for drawing, illustrating, presenting content in creative ways
- Have students draw definitions of constructive and destructive interference with waves or diffraction patterns of different shapes
- Engineering and design challenges connected to the content
- Create an experiment to measure the thickness of other objects (ex: a piece of wire, fishing wire, etc.)
Create an experiment to test different objects that will create a diffraction pattern (ex: two pencils really close together.
- MS-PS4-1Use mathematical representations to describe a simple model for waves that includes how the amplitude of a wave is related to the energy in a wave.
- MS-PS4-2Develop and use a model to describe how waves are reflected, absorbed, or transmitted through various materials.
- MS-PS4-3Integrate qualitative scientific and technical information to support the claim that digitized signals are a more reliable way to encode and transmit information than analog signals.
Credits
Coordinated, Researched and Written by: Rebecca Thompson-Flagg
Art Direction and Illustrations by Kerry G. Johnson
Created in collaboration with LaserFest 2010
Updated in 2023 by Sierra Crandell, M.Ed. partially funded by Eucalyptus Foundation
Extension by Jenna Tempkin with Society of Physics Students (SPS)