DISK-COVER X-RAY DIFFRACTION



Authors

  • RET Fellow: Evelyn Montalvo
  • RET Leadership Team: RET Teachers, Ben Taylor, George Lisensky, Paul Evans, Margaret Cosgriff and Anne Lynn Gillian-Daniel.

Audience

Students in grades 10 to 12 in a Chemistry or Physics course

Time Frame

The complete activity requires 100 minutes if Part 2 is done the same day. The activity can be divided into 50- min periods where Part 2 can be done the next day.
  • Set-up: 5 minutes
  • Activity:
    • Part 1: 40 minutes
    • Part 2: laboratory work 50 minutes
  • Clean-up: 5 minutes
Objective(s)

After completing the activity, students will be able to:
  1. Explain that X-Ray diffraction technology is a tool used to identify new materials, determine angles and bond lengths in crystal structures .
  2. Analyze the engineering behind materials such as CDs, DVDs and blu-ray disc that they encounter in everyday life.
  3. Apply mathematical formulas when they calculate experimentally the distance between the groove spacing or track pitch in a CD by using the Frauhofer equation (nλ= dsinθ) that is analogous to the Bragg equation (nλ=2dsinθ) which is the guiding principle for x-ray diffraction.

Standards Addressed
Next Generation Science Standards PS 4
  1. Construct explanations for why the λ of an electromagnetic wave determines its use for certain applications
  2. Make inferences and justify conclusions from sample experiments.
  3. Analyze that multiple technologies are based on the understanding of waves and their interactions with matter are everyday experiences in the modern world 4.
  4. Reason quantitatively and use units to solve problems
National Science Education Content Standards
  1. Students relate and apply concepts of energy and its interaction with matter to recognize types of waves and explain how they transfer energy, and to explain how wavelengths can be used to identify samples of matter.
  2. Students manipulate, communicate and present data using appropriate statistics, mathematical models, and available technology

Engineering principles

Grade Band End Points for ETS2.B By the end of grade 12.
Modern civilization depends on major technological systems, including those related to agriculture, health, water, energy, transportation, manufacturing, construction, and communications. Engineers continuously modify these technological systems by applying scientific knowledge and engineering design practices to increase benefits while decreasing costs and risks. Widespread adoption of technological innovations often depends on market forces or other societal demands, but it may also be subject to evaluation by scientists and engineers and to eventual government regulation. New technologies can have deep impacts on society and the environment, including some that were not anticipated or that may build up over time to a level that requires attention or mitigation. Analysis of costs, environmental impacts, and risks, as well as of expected benefits, is a critical aspect of decisions about technology use.
HS-ETS-ETSS:
Analyze data to compare different technologies designed to accomplish the same function regarding their relative environmental impacts, costs, risks, and benefits, and what may need to be done to reduce unanticipated negative effects.
Activity Materials

To make better use of time in terms of distributing materials, it is suggested that kits are assembled before class.
Include in quart size plastic baggies the following materials:
  • 1 diffraction grating slide
  • 1 magnifying glass (option: if there are microscopes available, then use them to view the slide)
  • 1 CD, 1 DVD, 1Blu- ray disc All must be unidentified. ( how to remove identifiers from the disks will be explained at the additional information section)
  • A set of 3 lasers of different wavelengths; red, green and blue        
  • Measuring tape and or metric ruler
Activity Instructions

Set-up
Before class, prepare as many kits as you need according to the number of lab groups organized. The minimum amount of students in a group should be 3. Preparation of kits once you gather all of your materials should not take more than 10 minutes. Preparation of the disks require more time. You should do this before the kits are to be assembled. Blank disks are recommended but one can try with used ones as well.
CD = made up by one layer so you only have to strip the label with a strong adhesive Tape. Make sure that the aluminum layer is also removed. You end up with a clear ,transparent disc.
DVD = made out of two layers so splice the disk with a razor. Use the plastic, bluish color, transparent disk that comes out when you splice it
Blu-Ray = some are one layered and others are two layered. The one used in this activity is one layered. Remove the label by applying a coat of a soy based paint remover( more user friendly) and wipe label off. Repeat procedure if necessary. The disk will not be clear but it can be used for this activity
Introduction (20 min)
Prior to the activity and within the lesson of DNA in Biology or Crystal structures in Chemistry, students should understand how were scientists able to determine the DNA structure or, in chemistry, how unit cells, bond angles and bond lengths are determined. This is the section where you introduce the technology of X ray diffraction. (An organizational diagram is provided in the supplemental materials section.)
At this time provide the diffraction grating so they can further understand what the idea of a pattern is and how light behaves through these patterns.
The introduction of the law that is the basis for this technology (Bragg’s Law diagram provided) can be presented and explained so later on they will understand why is it that the upcoming activity is analogous to the concept of x ray diffraction (diagram is provided).
At this time provide the diffraction grating so they can further understand what the idea of a pattern is and how light behaves through these patterns by letting them observe with a magnifying glass or microscope. Now ask them to use the red laser to project the pattern of the diffraction grating slide. Make sure you have discussed the safety issues involved when using a laser BEFORE asking them to use it. They should be able to discover that the pattern observed is projected inversely and the closer the lines are the farther they are projected.
The Next Part of the Activity (40 min)
Present SEM images of a CD, DVD and Blu ray disk in a transparency or use a power point slide for this.(images provided in the supplemental material section) Do not tell them what they are but have them infer what they think they are and describe what they see. Ask them to compare and contrast what they see. If time runs too long and they do not figure them out, provide a hint such as: “they are related to recording information”. Someone will surely come up with CD or the names of the other disk. Begin a discussion on why they are different, their use and capabilities and have them infer which image belongs to each disk. Once they have established the correct matching provide the kits without telling them which disc is which. Since they already used the laser when viewing the diffraction grating sample, ask them if they could identify each disk by using it as a diffraction grating. Ask them to record everything they observe and use the 3 different lasers. Make sure that they are viewing the disks at the same distance as to avoid changing variables. The independent variable will be the type of laser (its wavelength) and the dependant variable will be the distance measured from the central point. Variables that should remain constant; the distance the disk is placed and the distance the laser is placed. They can also measure with a metric ruler the distance between the central point and the adjacent points if they are able to see them (NOTE: the blu ray only projects one central point so this one cannot be measured in this activity) As a group they will analyze their observations and justify with evidence. They can design a procedure since some might want to make measurements to help them decide.
Make sure the room is dark enough and that each team has access to a wall for proper projection. Repeat the safety concerns before carrying out this part. Have them report in groups or have them hand in their conclusions and justifications
The Third Part of the Activity (50 min)
For this part, the introduction to the Fraunhofer diffraction equation should be introduced. They should be able to analyze that this equation is analogous to the Bragg equation shown previously. (Diagram of equation provided in supplemental materials section).
Ask students that based on the variables present in the equation if they can design a procedure in which the distance between the grooves of a CD or DVD can be calculated. Provide the kit and let them choose if they want to work with the CD or DVD and select the laser of their choice. Once the teacher approves the procedure they can collect the data necessary and substitute in the formula.
Conclusion (homework and assessment teacher’s choice)
Once they have concluded their measurements and calculated the distance, provide the true value of the distance between the grooves of the disk chosen. Have them calculate percent error and justify it. This can be done as part of a complete lab report by the group or individual one or assigned for homework. The amount of points given to this activity is priority of the teacher. The true value of the CD is 1.6um and for the DVD is .78um
       
Background
  • Explanation of x rays, diffraction and how the technique is used in science and Engineering.
  • X ray diffraction must fulfill the Bragg equation for diffraction to occur. Familiarize with the Fraunhofer equation to carry out the laboratory portion of the activity. NOTE: IF the math portion of the activity deems too difficult or time consuming, the teacher can do the first two parts of this lesson only.
Supplemental Materials



References