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Becke Line Test Simulator

Run the Becke Line Test MicroSim Fullscreen

About This MicroSim

To identify a glass fragment, an examiner measures its refractive index (RI) by immersing it in oils of known RI. When the glass and oil RIs differ, a bright rim of light — the Becke line — appears at the boundary. As you change the focus (stage height), that line moves, and which way it moves tells you which medium has the higher RI:

  • Raise the stage → the Becke line moves into the medium with the higher refractive index.
  • Lower the stage → it moves into the lower-RI medium.
  • When glass and oil RIs are equal, the line disappears and the fragment becomes nearly invisible — that is the match.

This MicroSim recreates the microscope field of view so you can manipulate the RI difference and read the directional response.

How to Use It

  1. Set the Glass RI and Oil RI sliders (or pick a calibrated oil preset from the dropdown).
  2. Drag the Stage height slider above or below focus and watch the white Becke line shift into the glass or the oil.
  3. Read the Conclusion panel: Glass RI > Oil RI, Glass RI < Oil RI, or Match.
  4. Narrow in on a match by trying successive oils until the fragment vanishes.

What You Can Learn

  • Describe the Becke line test procedure for comparing refractive indices.
  • Interpret the Becke line's direction of motion as you change stage height.
  • Conclude whether the glass RI is greater than, less than, or equal to the oil.

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/becke-line-test/main.html"
        width="100%" height="512" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 15–20 minutes Bloom level: Understand (L2) — interpret.

Warm-up. Remind students that a higher RI bends light more. Pose: "If the glass bends light more than the oil, and we raise the focus, which way will the bright line move?"

Guided questions:

  • With Glass n = 1.520 and Oil n = 1.490, raise the stage. Into which medium does the Becke line move, and why?
  • How do you produce a "match," and what does the field of view look like then?
  • Why is the systematic narrowing process (trying several oils) more reliable than a single observation?

Extension. Discuss how temperature and wavelength affect oil RI, and why forensic labs control both during the test.

References

Specification

This MicroSim was generated from a specification in Chapter 5: Glass, Soil, and Physical Trace Evidence.

Design note: the optics are modeled qualitatively for teaching — the Becke line offsets toward the higher-RI medium when the stage is raised and fades within ±0.002 RI of a match. The fragment outline and vignette are stylized, not a ray-traced simulation. RI sliders and stage height use p5 DOM controls; an oil-preset dropdown drives the systematic narrowing process.