A Water Prism: Gentler Dispersion Than Glass

1 · Predict

Water has a lower refractive index (n ≈ 1.33) than glass. Does a water-filled prism deviate light less than an equivalent glass prism?

• Yes — water's lower index means less bending at each face, so less total deviation than a glass prism of the same apex angle.
• A water prism deviates light more than glass.
• Deviation is the same regardless of the medium's index.

2 · Set Up

1. Open the water-prism preset and press Reset. This water-filled prism has apex angle A = 60°, index n = 1.33.
2. Enable the total-deviation readout.
3. Set the incidence angle 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. Compare the scale to the white-light-prism (crown glass) experiment's.

4 · Analyze

1. For one trial, compute θ₂ = asin(sin θ₁/n), θ₃ = A − θ₂, θ₄ = asin(n·sin θ₃), then δ = θ₁ + θ₄ − A, using n = 1.33. Compare to the table.
2. Compare your deviation values to the white-light-prism experiment's (same apex angle, higher index). Explain why water's lower index produces smaller deviation at every matching incidence angle.

5 · Extend

1. A rainbow is essentially millions of tiny spherical water-droplet 'prisms' refracting and internally reflecting sunlight. Explain why water's relatively modest dispersion (compared to glass) still produces a visible, if somewhat less vivid, spectrum in a rainbow.
2. Across this experiment, flint-prism (n≈1.66), white-light-prism/crown glass (n=1.5), and water-prism (n=1.33) form a clear trend: denser optical media disperse and deviate light more. Explain why this trend follows the same physics as the tir experiment's critical-angle trend.

The Physics Behind This Experiment

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