Authentic Lessons for 21st Century Learning

Diffraction Unit, Lesson 1: Bending Light

What Is Diffraction?

Sherry Franklin, Lindsey Link, Ann Williams | Published: July 13th, 2022 by K20 Center

  • Grade Level Grade Level 9th, 10th, 11th, 12th, Secondary
  • Subject Subject Science
  • Course Course Chemistry, Physics


Students will learn that light is formed by waves and can change direction through diffraction. They will use pencils to build a spectroscope and make observations as to the color and size of light. Students will research wave diffraction and use what they learned to build and test a spectroscope.

Essential Question(s)

What is diffraction? How does light bend when it passes through an object?



Students watch a phenomenon and describe diffraction.


Students explore how light can bend around edges through a pencil investigation.


Students complete an I Think/We Think chart, along with questions they still have, while watching a video on wave diffraction. Working in pairs, students choose 1–2 questions to add to the Driving Question Board.


Students research how to design and build a spectroscope, as well as one of the questions from the Driving Question Board.


Students test and revise their spectroscope.


  • Lesson Slides (attached)

  • I Think/We Think handout (attached; one per student)

  • Light Can Bend Around Edges! handout (attached; one per student)

  • Spectroscope Engineering handout (attached; one per pair of students)

  • Optional Engage Activity handout (attached; for teacher use)

  • Student laptops or tablets for research (one per pair of students)

  • Clean, new pencils with erasers (two per student)

  • Transparent tape (one piece per student)

  • Flashlight: Mini Maglite (one per student or group of students)

  • Optional: Pieces of cloth, a feather, plastic diffraction grating, metal screen, a human hair (one per student)

  • CDs or diffraction grating (one per pair of students)

  • Cereal boxes, paper towel tubes, or anything similar for the body of the spectroscope (one per pair of students)

  • Exacto knife (one per pair of students)

  • Tape for building spectroscopes (amount varies)

  • Any other materials needed/desired for building spectroscopes


Introduce the lesson using slide 2 of the attached Lesson Slides.

Display slide 3 to share the essential questions and slide 4 to go over the lesson's learning objectives to the extent you feel necessary.

Move to slide 5. Share the Using Phenomena to Drive Science Instruction strategy with students and preview the questions ahead of time.

Move to slide 6 to share the "Homemade Kaleidoscope" video from Sick Science (stop the video at 0:57).

Go to slide 7 to use the following questions to help guide a whole class discussion:

  • What is happening to the light inside the tube?

  • Why do these phenomena happen?

  • What is causing the colors to appear?

  • Do the colors always occur in the same order?


Display slides 8, and share that light bends when it passes around an edge or through a slit. This bending is called diffraction. You can easily demonstrate diffraction using a candle or a small bright flashlight bulb and a slit made with two pencils. The diffraction pattern—the pattern of dark and light created when light bends around an edge or edges—shows that light has wavelike properties.

Move to slide 9 and pass out the attached Light Can Bend Around Edges! handout. Have students gather all the material they need to complete this pencil activity from Exploratorium.

Move to slide 10 and walk students through properly assembling their materials.

Move to slide 11 and have students explore how light can bend around edges while completing the questions on the handout.

Once students are wrapping up their observations, share with them that the black bands between the blobs of light show that a wave is associated with the light. The light waves that go through the slit spread out, overlap, and add together, producing the diffraction pattern you see. Where the crest of one wave overlaps with the crest of another wave, the two waves combine to make a bigger wave, and you see a bright blob of light. Where the trough of one wave overlaps with the crest of another wave, the waves cancel each other out, and you see a dark band.

The angle at which the light bends is proportional to the wavelength of the light. Red light, for instance, has a longer wavelength than blue light, so it bends more than blue light does. This different amount of bending gives the blobs their colored edges: blue on the inside, red on the outside.

The narrower the slit, the more the light spreads out. In fact, the angle between two adjacent dark bands in the diffraction pattern is inversely proportional to the width of the slit.

Thin objects, such as a strand of hair, also diffract light. Light that passes around the hair spreads out, overlaps, and produces a diffraction pattern. Cloth and feathers, which are both made up of many smaller, thinner parts, produce complicated diffraction patterns.

Move to slide 12 and briefly review the difference between reflection, refraction, and diffraction.


Move to slide 13, share the I Think/We Think strategy with students, and pass out the attached I Think/We Think handout.

Move to slide 14 and have students watch the "Wave Diffraction" video. Have students fill out the first two columns of the handout while watching.

  • In the column labeled "I Think," record your observations.

  • In the column labeled "Questions," record any questions you have during the video.

Move to slide 15. Have students work with a partner and discuss the following:

  • Observations they both made during the video.

  • Write a one-sentence summary of their discussion and add it to the column labeled "We Think."

  • Select 1–2 questions that they both still have to share with the class.

Once all groups have finished, move to slide 16. Share the Driving Question Board strategy with students. Based on what they discussed with their partner, ask them to share some questions that can be added to the Driving Question Board. These questions will be used in the next part of the lesson.


Move to slide 17 and tell students that their mission is to design, build, and refine a spectroscope to observe light spectra. Preview what they will be doing during this time:

  • Select one question from the Driving Question Board that they would like to research.

  • Watch the videos to learn more about how to create a spectroscope.

  • Design and build a spectroscope using diffraction materials.

  • Then, they will show off the spectroscope to other groups in a Gallery Walk.

Move to slide 18 and pass out the attached Spectroscope Engineering handout. Explain to students that, as they watch the videos that explain the science behind spectroscopy, they need to keep track of their research and notes on the handout.

Move to slide 19. Play the "What's Going On: CD Spectroscope" video and have students take notes on the handout.

Move to slide 20. Remind students:

  • Their mission is to design, build, and refine a spectroscope using diffraction material to observe light spectra.

  • They will need to watch at least two of the provided videos on the Spectroscope Engineering handout and possibly find some of their own for inspiration.

Move to slide 21 and have student pairs share what they learned from their research on their Driving Question Board question.

Move to slide 22 and remind students that they need to create a spectroscope that meets the following criteria:

  • A slit

  • A part to diffract

  • A place to look through

When ready, have students gather their materials and start building.

Move to slide 23 and share the Gallery Walk strategy with students. As they walk around the room to view the spectroscopes built by their peers, ask them to take time to notice at least two similarities and at least two differences. Remind students to be prepared to share with the class.

After the Gallery Walk, take some time for students to discuss what they discovered while viewing other students' spectroscopes.


Move to slide 24 and have students revisit the Spectroscope Engineering handout. Have students complete the Build and Test Your Spectroscope section of the handout as they are outside looking at light and clouds. This activity should take 10–15 minutes depending on your class.

Guide students in a class discussion about what they observed, what they might change, and what worked well while using their spectroscopes.

Possible discussion questions:

  • Identify at least one refinement to your spectroscope. How did you modify your design after you started building to improve performance? Explain how the refinement improved the performance.

  • If you were asked to build another spectroscope, name at least two things that worked well and you would do again.

  • If you were asked to build another spectroscope, name at least two things you would do differently.

Move to slide 25 and have students complete the Refine and Reflection questions on the Spectroscope Engineering handout.

Move to slide 26. Remind students of the essential questions from the start of the unit. Have students answer the essential questions:

  • What is diffraction?

  • How does light bend when it passes through an object?


AfterschoolUniverse. (2011). Afterschool universe: Paper towel tube spectroscope [Video]. YouTube.

Bozeman Science. (2015). Wave diffraction [Video]. YouTube.

CD spectroscope: Introduction. (2015). Exploratorium.

Diffraction. (n.d.). Exploratorium.

Exploratorium Teacher Institute. (2015). What's Going On: CD Spectroscope - Science Snacks Activity [Video]. YouTube.

K20 Center. (n.d.). I Think/We Think. Strategies.

K20 Center. (n.d.). Driving Question Board. Strategies.

K20 Center. (n.d.). Using Phenomena to Drive Science Instruction. Strategies.

K20 Center. (n.d.). Gallery Walk. Strategies.

Noll, M. (2015). Making your Foldable Paper Cellphone Spectrometer. YouTube.

Public Lab. (n.d.). Foldable mini-spectrometer. Public Lab.

Sick Science! (2014). Homemade Kaleidoscope- Sick Science! #179 [Video]. YouTube.