A Prism Bends Light: Angle of Deviation

1 · Predict

Light entering a glass prism bends (refracts) at each face, emerging deviated from its original direction. Does changing the incidence angle at the first face always change the total deviation in the same direction?

• No — deviation actually has a minimum at some specific incidence angle, not a simple increasing or decreasing trend.
• Deviation always increases as incidence angle increases.
• Deviation always decreases as incidence angle increases.

2 · Set Up

1. Open the white-light-prism preset and press Reset. This crown-glass prism has apex angle A = 60°, index n = 1.5.
2. Enable the total-deviation readout.
3. Set the incidence angle at the first face for each trial and record the total deviation.

3 · Collect Data

Plot deviation δ (y-axis) against incidence angle θ₁ (x-axis) for your three trials. Does δ dip to a minimum somewhere in the middle?

4 · Analyze

1. For one trial, compute θ₂ = asin(sin θ₁/n) (Snell at the first face), θ₃ = A − θ₂ (prism geometry), θ₄ = asin(n·sin θ₃) (Snell at the second face), then δ = θ₁ + θ₄ − A. Compare to the table.
2. Explain why deviation isn't a simple monotonic function of incidence angle — the prism's geometry (bending at TWO faces) creates a genuine minimum at the angle of symmetric passage, explored directly in the min-deviation experiment.

5 · Extend

1. White light contains many wavelengths, each with a slightly different refractive index (dispersion) — so each color deviates by a slightly different amount, fanning white light into a visible spectrum. This experiment uses a single representative index; real white light spreads into a rainbow.
2. A rainbow forms from sunlight refracting and internally reflecting inside spherical raindrops — a more complex geometry than this flat-faced prism, but driven by the exact same wavelength-dependent refraction (dispersion) principle.

The Physics Behind This Experiment

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