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Diffraction of Light
July 2005
(Original concept – Cornell Center for Material Research)
CIPT 5E’s Modification: Andrew J. Telesca, Jr., Martin Alderman, Jim
Overhiser
Physics: Grades 9-12
The diffraction and interference of light are easily observed phenomena
supporting the wave nature of light. Diffraction is at the root of many
technologies, scientific techniques, and common visual phenomena. The
students will be engaged by examining some single slit diffraction and
considering some questions about single slit diffraction. They will have a
chance to explore by doing several hands-on activities related to diffraction and
interference. What they observed will be explained in a discussion of the
hands-on activities and associated questions. Elaboration will involve the
students involvement in challenge question that relate to the content of the
activities. The students will have a chance to use their newly acquired
understandings in the evaluation phase.
Two 40 minute periods
Standard 1, Math Analysis, Key Idea 3, Scientific Inquiry Key Idea 1
Standard 4, Key Idea 4, Performance Indicator 4.3, Major Understandings l,
m.4.3h When a wave strikes a boundary between two media, reflection,
transmission, and absorption occur. A transmitted wave may be refracted.
CIPT – Diffraction and Interference of Light (5 E’s lesson plan.)
ENGAGE
Materials:
• 2 standard #2 pencils
• small rubberbands
Activity: The students will attach the two pencils together using the rubberbands supplied.
Looking at a light source through the slit between the pencils students will observe a series of
thin lines.
Instruction: Instruct the students in the construction of the pencil-rubberband device. They may
improve their viewing by changing the slit width. This can be done by squeezing the pencils to
decrease the slit size or by pulling them gently apart to increase the size.
Without explaining the science, ask the students:
“What did you see?”
“What do you think causes this?
“Have you ever seen anything like this?”
“Can you create the same effect using two of your fingers?”
EXPLORE
Experience 1:
Materials:
• pen LASER
• 2 diffraction grating (500 lines/mm, 1000 lines/mm)
Activity: Students will shine the LASER light through the diffraction grating and observe both
the effect of the gratings on the LASER light but also the difference in the effect between the two
different gratings.
Instructions: Shine a pen LASER light through each grating slide onto a distant, flat surface.
The gratings are made up of a series of very closely spaced strips.
Carefully sketch the patterns you see. Change the distance between the grating and the
projecting surface and observe the any change.
Experience 2:
Materials:
• pen LASER
• fine and coarse wire mesh
Activity: Students will shine a LASER light through fine wire mesh and coarse wire mesh and
make observations of the diffraction patterns.
Instructions: Holding the fine mesh approximately 1 meter from a screen (wall), shine the
LASER through the fine mesh on to the screen. Observe the pattern and the spacing of what is
produced. Repeat this process using the coarse mesh and record the results.
Have students respond to the following:
Describe the difference between the pattern produced by the fine mesh and the pattern
produced by the coarse mesh.
Compare the pattern produced with the mesh to the pattern produced by the diffraction
grating seen in Experience 1.
Make a prediction as to the appearance of a medium mesh. How do you justify your
prediction?
Experience 3:
• Pen LASER
• LASER holding block
• Human hair
• Tape
• Blank photographic slide
• Ring stand w/ test tube clamp
• Microscope (optional)
Activity: Students will use a single human hair to create a diffraction pattern. Students will then
compare thick vs fine hair piece.
Instructions:
Secure a piece of human hair and tape it across the opening of a blank
photographic slide. Support the slide in a test tube clamp and place the LASER in the support
block. Adjust the LASER to strike the hair directly and look at the image produced. (Note:
This activity works best in a darken room and at a distance of greater than 1 meter.)
Repeat this experiment using hairs of varying thickness. (Note: A microscope may be used to
make a better observation on relative hair thickness).
Have students respond to the following:
Describe the difference between the varying hair thicknesses.
Are there any similarities/differences between the patterns created with human hair and the
patterns you have observed so far in other activities in this lab?
Experience 4:
Materials:
• Penlight
• Blank CD and DVD
• Ring stand and test tube clamp
• Microscope (optional)
Activity: Students will shine a flashlight on a CD and a DVD and compare the diffraction
patterns produced by each.
Instructions: Using the set-up pictured below, adjust the height to the flashlight and observe the
differences in the diffractions patterns in the CD vs the DVD.
Have the students respond to the following:
Describe what you see when the light is ‘reflected’ from the CD and DVD.
How do you think the recording surfaces of the CD and the DVD differ?
How could you determine the physical difference between these recording surfaces?
Note: For more student activities, refer to Abbreviated Teacher Note Section: Extra
Experiments for Students section.
EXPLAIN
Materials:
• None
Activity: Students will participate in a discussion of the commonalities sound in the activities.
The teacher will direct the discussion to lead the students toward an understanding of the
following:
Diffraction
Huygen’s Principle
Babinet’s Principle
Wave interference
Coherent light
LASER
Instructions: Teacher leads a discussion using the following:
Discuss the EXPLORE question responses.
“Describe all of the things that are similar in all of the four EXPLORE activities and the
ENGAGE activity.”
Before presenting students with formal definitions, perform the following demonstrations:
o DEMO: Wave tank (show wave properties of diffraction through a narrow slit;
(define DIFFRACTION and HUYGEN’S PRINCIPAL).
o DEMO: Sound wave walk (Set up a pair of stereo speakers to a sound wave
generator and as the speakers are producing the tone, have the students walk
between the speakers to hear the increase and decreases in volume = interference
patterns in the sound.); (define INTERFERENCE)
What about the piece of hair? It is not a slit, so how does it work? (define BABINET’S
PRINCIPLE)
ELABORATE
Materials:
• CD blank disc
• DVD blank disc
• HeNe LASER
• Ringstand w/test tube clamp
• Metric ruler
• Calculator
Activity: Students will determine the spacing between tracks on a CD and DVD (also known as
pitch) using Young’s equation for double-slit diffraction.
Instructions: (See Abbreviated Teacher Note Section)
EXPLORE
Materials:
• Lycopodium Powder
• Microscope slide
• Metric ruler
• HeNe LASER
Activity: Students will work in small groups to solve the problem: “What is the approximate
size of a lycopodium spore?” Students will report-out on the results of their group’s experiment
using white boards.
Instructions: Because this is an evaluation, instructor involvement should be minimal. The
teacher should monitor students progress and make certain they are using the rings in the
lycopodium diffraction pattern as the bright lines seen in a slit experiment.
CIPT – Diffraction and Interference of Light
Student Worksheet
Pencil/Rubberband activity
Materials:
• 2 standard #2 pencils
• small rubberbands
Instruction: Construct a pencil-rubberband device. This is done by taking two pencils and
stretching a rubberband around the top and bottom of the two pencils. Look at a distant light
source through the narrow slit and record observation on what you see. You should see a series
of lines. You may improve your viewing by changing the slit width. This can be done by
squeezing the pencils to decrease the slit size or by pulling them gently apart to increase the size.
Respond to the following questions:
“What did you see?”
“What do you think causes this?
“Have you ever seen anything like this?”
“Create the same effect using two of your fingers. Describe how you did this successfully”
Experience 1:
(Diffration Gratings)
Materials:
• pen LASER
• 2 diffraction grating (500 lines/mm, 1000 lines/mm)
Instructions: Shine a pen LASER light through each grating slide onto a distant, flat surface.
The gratings are made up of a series of very closely spaced strips.
Carefully sketch the patterns you see. Change the distance between the grating and the
projecting surface and observe the any change.
OBSERVATIONS:
500 lines/mm
1000 lines/mm
What affect did changing the distance between the grating and the wall have on the image
produced?
Experience 2:
(Wire mesh)
Materials:
• pen LASER
• fine and coarse wire mesh
Instructions: Holding the fine mesh approximately 1 meter from a screen (wall), shine the
LASER through the fine mesh on to the screen. Observe the pattern and the spacing of what is
produced. Repeat this process using the coarse mesh and record the results.
Record your observations in the spaces below:
Fine mesh
Coarse mesh
Respond to the following:
Compare the pattern produced with the mesh to the pattern produced by the diffraction grating
seen in Experience 1.
Make a prediction as to the appearance of a medium mesh. How do you justify your prediction?
Experience 3: (Human Hair)
• Pen LASER
• LASER holding block
• Human hair
• Tape
• Blank photographic slide
• Ring stand w/ test tube clamp
• Microscope (optional)
Instructions:
Secure a piece of human hair and tape it across the opening of a blank
photographic slide. Support the slide in a test tube clamp and place the LASER in the support
block. Adjust the LASER to strike the hair directly and look at the image produced. (Note:
This activity works best in a darken room and at a distance of greater than 1 meter.)
Repeat this experiment using hairs of varying thickness. (Note: A microscope may be used to
make a better observation on relative hair thickness).
Respond to the following:
Describe the difference in the pattern created between the varying hair thicknesses.
Are there any similarities/differences between the patterns created with human hair and the
patterns you have observed so far in other activities in this lab?
Experience 4: (CD and DVD Diffraction)
Materials:
• Penlight
• Blank CD and DVD
• Ring stand and test tube clamp
• Microscope (optional)
Instructions: Using the set-up pictured below, adjust the height to the flashlight and observe the
differences in the diffractions patterns in the CD vs the DVD.
[JPEG of set up]
Respond to the following:
Describe what you see when the light is ‘reflected’ from the CD and DVD.
How do you think the recording surfaces of the CD and the DVD differ?
How could you determine the physical difference between these recording surfaces?
You will now participate in a discussion of the commonalities found in these activities. The
teacher will direct the discussion to lead the students toward an understanding of related physics
concepts. As you listen to the discussion, develop a personal definition for the following terms
and record them in the spaces below.
Diffraction
Huygen’s Principle
Babinet’s Principle
Wave interference
Coherent light
LASER
Respond to the following questions:
Describe all of the things that are similar in all of the five activities performed so far.
What concept(s) did the wave tank demonstration help you to understand?
What concept(s) did the sound walk demonstration help you to understand?
What about the piece of hair? It is not a slit, so how does it work?
Challenge: What is the spacing difference between the lines of a CD vs a DVD?
Materials:
• CD blank disc
• DVD blank disc
• HeNe LASER
• Ringstand w/test tube clamp
• Metric ruler
• Calculator
Instructions: (See hand-out)
Distance between lines:
CD ______________
DVD ______________
Challenge: What is the approximate size of a lycopodium spore?
Materials:
• Lycopodium Powder
• Microscope slide
• Metric ruler
• HeNe LASER
Describe your experiment in the space below: (Illustrations are a good thing)