IB Physics IA idea · Light and optics
IB Physics IA idea: use a CD as a grating to measure its track spacing
The rainbow on a disc comes from a grating of microscopic grooves. Use it to measure them.
These ideas are starting points, not an IA to copy. Never copy an idea blindly: make the question, method and analysis your own, and check your plan with your teacher.
Example research question
How does the angle of the first-order beam reflected from a CD depend on the angle of incidence of a class 2 laser from 0° to 50° in eight steps, and what track spacing follows from the reflection grating equation?
Change the system, the range or the method so the question is yours. Check it against the research question checklist in our Physics IA guide.
Variables
- You change
- angle of incidence of a class 2 laser on the disc, 0 to 50 degrees in 8 steps
- You measure
- angle of the first-order reflected beam, 0 to 70 degrees
- You keep the same
- laser wavelength (one pointer)
- distance to the screen
- disc flatness
- alignment of the plane of incidence
Method
Reflect a class 2 laser from the disc at several angles of incidence and locate the first-order spot on a screen; compute the angles from distances. Repeat with a second wavelength for a check.
Data analysis
Grating equation for reflection: d(sin theta_i - sin theta_m) = lambda. Plot sin theta_m against sin theta_i; the gradient should be 1 (a test of the model) and the intercept is -lambda/d, giving the track pitch. Compare with 1.6 micrometres; use error bars from screen positions and max/min lines.
How it works
You change Angle of incidence (degrees) and measure First-order beam angle (degrees) in Laser and CD, keeping Laser wavelength, Screen distance, Disc orientation the same. Expect a graph of First-order angle / ° against Angle of incidence / °: The beam moves as the disc tilts.
Make it yours
Test a DVD (0.74 micrometre) or a Blu-ray and compare.
Safety
Take care with: lasers.
Class 2 lasers; avoid reflected beams.
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