Unit 4 - Day 9: Single-Slit Diffraction
Course/Section: SPH4U/Wave Nature of Light
Date: To be decided
Lesson Big Idea(s):
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Light has properties that are similar to those of mechanical waves.
The behavior of light as a wave can be described mathematically.
Ministry Expectations:
E2.3 - conduct inquiries involving the diffraction, refraction, polarization, and interference of light waves.
E2.4 - analyze diffraction and interference of water waves and light waves.
E3.2 - describe and explain the diffraction, refraction, polarization, and interference of light waves.
E3.3 - use the concepts of refraction, diffraction, polarization, and wave interference to explain the separation of light
into colors in various situations.
Learning Goals:
Success Criteria:
By the end of the class students will be able to:
 Explain how single-slit diffraction creates interference
patterns to increase the resolution of images.
I know students have achieved their learning goals when
they can:
 Describe and explain the diffraction pattern of light
through a single-slit.
 Mathematically solve problems involving a single-slit
interference pattern.
 Describe resolution in terms of diffraction of light.
Expected Prior Knowledge:
 Huygens’ Principle: each point on a wave front acts as a new point source for a spherical wave, the evolution of the
wave front can be determined by superposing all of these spherical waves at later times.
Before: Minds On
Time:
5 min
Description
Materials
Minds On: Producing an interference pattern with your fingers!
 Fluorescent lights
 Have hold their index and middle fingers, about 2mm apart in front of their eyes
and then look at the fluorescent lights in the room. What do you see?
 Facilitate a class discussion.
Transition from Minds On to Action:
 Now that we’ve seen that it is possible to produce an interference pattern using a single-slit, we’ll begin to
quantify this phenomenon.
During: Action
Time:
Description
15 min
Action 1: What Causes Single-Slit Diffraction?
 Show single-slit diffraction (i.e. Frauhofer Diffraction) video from MIT Open Course
Ware: http://www.youtube.com/watch?v=PgW7qaOZD0U
 Discussion Question: If there is only one slit, what is causing the interference
pattern?
 Allow students some time to discuss the answer to the above question in their
groups and write down ideas on whiteboards. The responses will then be shared
with the class.
 Discuss Huygens’ principle: all points across a slit act as wave sources, and these
different waves create an interference pattern on the screen.
 We have already used Huygens’ principle to discuss refraction and diffraction.
Light ways that diffract are coherent and interfere.
 The interference pattern that formed by the diffracted light is called the
diffraction pattern.
Materials
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Laptop
LCD projector
Internet
Whiteboards
Markers
Unit 4 - Day 9: Single-Slit Diffraction
40 min
Action 2: Mini Investigation - Diffraction of Light Using a Single-Slit
 See materials in
lab handout
 See separated handout titled Single-Slit Diffraction Lab.docx
 Copies of Single Students will have seen how the lab is performed in the video from Action 1.
Slit Diffraction
 Have students record their results on the blackboard after they’ve completed their
Lab
portion of the activity.
After: Consolidation
Time
15 min
Description
Materials
Consolidation: Summary
 Blackboard
 Discuss proportionality relationships from student’s results, i.e. the distance  Chalk
between adjacent maxima is proportional to the wavelength of light and inversely
proportional to the slit spacing.
 Use their results to develop a formula for the slit spacing, which is consistent with
the observed proportionality relationships discovered in class, specifically:
Dy =
lD
w