Investigating Science 11–12 · Year 12

Diffraction as a measuring tool: a CD, a laser and an X-ray diffraction analogue

Module 6: Technologies

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

A regular pattern too fine to see spreads light into a predictable pattern, so measuring the pattern reveals the spacing; this is the principle that let X-ray diffraction reveal the helical structure of DNA.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

eye injury from a direct or mirror-like reflected laser beam

Controls

keep the beam below eye level and pointed at the screen
remove watches and reflective objects from the beam path
only the teacher or a supervised learner switches the laser on

Note

No chemicals or heat. A laser is used: follow the school’s laser procedure (ASTA standard operating procedure, Use of lasers in schools) and record the activity in RiskAssess.

What you need

a Class 2 red laser pointer (650 nm, output below 1 mW)
a CD and a DVD, a pen spring stretched slightly and clamped, white screen, metre rule, retort stand

How to do it

Clamp the laser at bench height and shine it perpendicularly onto the CD so the reflected spots fall on the screen.
Measure the CD-to-screen distance L and the distance x from the central spot to each first-order spot.
Calculate θ = arctan(x/L) and the track spacing d = λ/sin θ.
Repeat with the DVD.
Shine the laser through the stretched pen spring onto the screen and sketch the pattern; stretch the spring further and record the change.
Relate the spring pattern to X-ray diffraction images of DNA and to the development of technology that made the images possible.

What you should see

The CD standard (ECMA-130) sets a track pitch of 1.6 ± 0.1 µm, so a 650 nm laser gives first-order spots at 24.0° (22.5 to 25.7° across the tolerance); at L = 0.500 m they sit 0.222 m from the centre. The DVD’s 0.74 µm pitch gives 61.4°. The learner compares their calculated CD spacing with the standard’s tolerance. The spring gives a cross-shaped pattern whose angle changes as the spring is stretched, the signature of a helix in a diffraction image.

What changes

What you change: grating (CD or DVD) and spring stretch
What you measure: diffraction angle and pattern shape
What you keep the same: same laser wavelength
beam perpendicular to the disc
distance to screen measured each time

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The colours on a CD come from a coating or dye.
Diffraction needs openings about the size of the light beam.
An X-ray image of DNA is a photograph of the molecule.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS12-13 INS11/12-1 INS11/12-2 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities