Optics
Water prism (rainbow)
Water (n≈1.33) disperses far less than glass — the gentle spread that paints a rainbow.
Water prism (rainbow) — interactive Optics simulation. Water (n≈1.33) disperses far less than glass — the gentle spread that paints a rainbow. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Water dispersion
Water (n≈1.33) disperses far less than glass — the gentle spread that paints a rainbow.
Water's modest index (≈1.33) and low dispersion produce a gentle spectrum—the physics behind natural rainbows where tiny water droplets act as prisms. Less spread than glass but enough to separate colors visibly.
- n ≈ 1.33
- Gentle rainbow spread
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
Water has a lower refractive index (n ≈ 1.33) than glass. Does a water-filled prism deviate light less than an equivalent glass prism?
Predictions to weigh
- 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.
Variable roles
What you set:
- Incidence angle θ₁ (°)
What you measure:
- Total deviation δ (°)
How the investigation runs
- Open the water-prism preset and press Reset. This water-filled prism has apex angle A = 60°, index n = 1.33.
- Enable the total-deviation readout.
- Set the incidence angle for each trial and record the total deviation.
Governing equation
Lower-Index Media Deviate Less — δ = θ₁ + θ₄ − A
Water's refractive index (1.33) is lower than typical glass (1.5–1.66), so a water prism bends light less strongly at each face — smaller total deviation δ = θ₁ + θ₄ − A for the same apex angle and incidence.
What the printable worksheet asks students to work out
- 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.
- 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.
Where this shows up beyond the lab
- 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.
- 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.
- AP Physics 2 — Unit 13: Geometric Optics
- General High School Physics — Light & optics
- NGSS High School Physics — Wave properties
- Welcome to Water Prism
- Select the prism
- Press Play
- Gentle spread in water
- 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.