Unit 4 - Day 10: The Diffraction Grating
Course/Unit: SPH4U / Wave Nature of Light Date: To be decided
Lesson Big Idea(s):

The maxima seen in the double slit experiment can be narrowed to more precise peaks with the addition of many more slits.

Diffraction gratings are used to separate colours from an incoming spectrum. This is used in spectroscopy.
Overall Expectations:
• Perform experiments relating the wave model of light and technical applications of electromagnetic radiation (e.g., lasers and fibre optics) to the phenomena of refraction, diffraction, interference, and polarization.
• Analyse phenomena involving light and colour, explain them in terms of the wave model of light, and explain how this model provides a basis for developing technological devices.
Specific Expectations:
Identify the interference pattern produced by the diffraction of light through diffraction gratings, and analyse it in qualitative and quantitative terms.
Learning Goals:
By the end of the class you will understand:

How double slit interference can be expanded to many

 slit interference
How the diffraction grating can be used to separate
EM radiation of different wavelengths
How diffraction gratings are used in technology, such as applications in disc encoding and spectroscopy
Success Criteria:
I know I have achieved the learning goals when I can:

Calculate the angles of maxima produced by a

 diffraction grating, given a particular wavelength
Determine the wavelength of light, given the positions of the maxima
Match the spectrum of a light source to a known atomic spectrum
Expected Prior Knowledge:

Wave interference

Single and double slit diffraction

The electromagnetic spectrum

Atomic structure (basic chemistry)
Time:
15 min
Description
Minds On: Diffraction grating demonstration
 Students will walk into class to find pieces of old CD’s on their tables. They will talk with each other, trying to guess the meaning of the discs while the teacher takes time for housekeeping (2 minutes).


Ask students to recall the double slit diffraction pattern. Does the pattern depend on wavelength? What will happen if more slits are added?
More slits  more paths of light converging on any given point  point of

 total constructive interference becomes more precise.
Diffraction gratings have O[10,000 slits / cm], giving sharp maximum peaks.
Demonstrate using diffraction grating placed on projector. Class should be able to see resultant rainbow on either side of the center. Demonstrate the
 effect of different coloured filters placed over the bulb.
The lesson will be focused on transmission gratings, however the CD’s on their tables are examples of reflective gratings.
 Questions?
Transition from Minds On to Action:

Now it’s your turn to play around with diffraction gratings . . .
Materials
 Projector, diffraction grating, light filters

Unit 4 - Day 10: The Diffraction Grating
Time: Description
~35 min
~20 min
Action1: Verify the formula for maxima positions

Students will work in groups of four for the whole period. Each student will be given safety goggles and a laboratory activity sheet.

Students will first verify the formula for positions of maxima, using a light of known wavelength.

Students will then use the formula to find out the wavelength of a mystery laser.

These lasers will be numbered and the teacher will have a list of wavelengths corresponding to the numbers.
These first two activities should work fairly well in a lit room, but the third activity will not. When students start to work on the third activity, half of the room should be dark while the other half remains light.
Per Group: goggles (per person), protractor, laser of known wavelength, mystery laser, diffraction grating, lamp, coloured pencils, pictures of atomic spectra
Action 2: Solve for the element

This is not a precise experiment, as spectrometers are too expensive for most public schools. A teacher who wishes to use spectrometers for a more quantitative activity could look into borrowing from a university.

This activity is qualitative. Students will observe the spectrum from a particular gaseous element and sketch their observations. They will compare this with pictures of known spectra to find out which element is in their lamp. The lamps will be numbered and the teacher will have a list of elements corresponding to
 the numbers.
Differentiated instruction: stronger students can be given lamps with more than one element.
Time Description
5 min
Consolidation: Hand-in laboratory exercises to be marked (on per student)

HW: Nelson pg 525 Q 1 – 9 & Compare the spectrum produced by a diffraction grating with the spectrum produced by a prism.