Optics
Minimum deviation
At symmetric passage the deviation is least — the classic way to measure a glass's index.
Minimum deviation — interactive Optics simulation. At symmetric passage the deviation is least — the classic way to measure a glass's index. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Minimum deviation
At symmetric passage the deviation is least — the classic way to measure a glass's index.
When the ray path through the prism is symmetric, deviation is minimized and the geometry simplifies for measuring refractive index. Spectrometers often adjust to this angle for accurate n(λ) determination.
- Symmetric passage
- Measure index n
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
A ray through a prism achieves its smallest possible deviation angle at one special incidence angle — symmetric passage, where the ray inside the prism travels parallel to its base. Does a wider-apex prism have a larger minimum deviation?
Predictions to weigh
- Yes — a bigger apex angle bends the ray more overall, giving a larger minimum deviation.
- A bigger apex angle gives a smaller minimum deviation.
- Minimum deviation doesn't depend on the apex angle.
Variable roles
What you set:
- Apex angle A (°)
What you measure:
- Minimum deviation δ_min (°)
How the investigation runs
- Open the min-deviation preset and press Reset. This 60°/n=1.5 prism is set to its symmetric-passage incidence angle, which minimizes deviation.
- Enable the deviation readout.
- Set the apex angle for each trial (index n = 1.5 fixed), then rotate the prism — drag the tilt slider — until the deviation readout stops falling and starts rising again. Record that smallest value: it is the minimum deviation for that apex.
Governing equation
Minimum Deviation Formula — n = sin((A+δmin)/2) / sin(A/2)
At the incidence angle giving symmetric passage (ray parallel to the base inside the prism), deviation reaches its minimum: δ_min = 2·asin(n·sin(A/2)) − A. This special angle is used experimentally to measure a prism's refractive index precisely.
What the printable worksheet asks students to work out
- For one trial, compute δ_min = 2·asin(n·sin(A/2)) − A using n = 1.5. Compare to the table.
- Explain why symmetric passage (the ray traveling parallel to the prism's base inside the glass) gives the smallest possible deviation for a given apex angle and index — any other incidence angle produces MORE deviation, not less.
Where this shows up beyond the lab
- The minimum-deviation formula can be inverted to MEASURE a glass's refractive index from an apex angle and a measured minimum deviation: n = sin((A+δ_min)/2)/sin(A/2). This is the classic prism-spectrometer technique. Why might finding the minimum experimentally (by slowly rotating the prism and watching for where deviation stops decreasing) be more reliable than measuring at an arbitrary incidence angle?
- A larger apex angle prism spreads white light into a wider spectrum. Explain, using your δ_min-vs-A relationship, why spectrometer designers might choose a larger apex angle when they need finer wavelength resolution.
- AP Physics 2 — Unit 13: Geometric Optics
- General High School Physics — Light & optics
- NGSS High School Physics — Wave properties
- Welcome to Min Deviation
- Select the prism
- Press Play
- Symmetric passage
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.