Through the Prism

Why it matters

A slab of glass shifts light sideways but lets it leave parallel. A PRISM refuses — it turns the beam permanently. That turn has a hidden minimum.

The idea in plain words

The prism's geometry in one line. Tap the terms.

δ = i + e − A

A = 60°, i = 48°, e = 49.2°. Find δ.

δ = 37.2° — and because i ≈ e, this is nearly minimum deviation

• degrees: δ — angle of deviation — how far the prism turns the ray
• degrees: i — angle of incidence at the first face
• degrees: e — angle of emergence at the second face
• degrees: A — angle of the prism (between the two refracting faces)
• δ = i + e − A = 48 + 49.2 − 60

Predict first

As you increase the angle of incidence on a prism from 30° to 80°, the deviation δ…

δ traces a U-shaped curve. At the bottom — minimum deviation — the ray passes symmetrically through the prism.

• first decreases, reaches a minimum, then increases — correct
• keeps increasing
• stays constant — the prism is fixed

What you do

Sweep the incidence angle and trace the δ–i curve point by point. Land the marker exactly on the bottom of the U.

Check yourself

At minimum deviation, the ray inside the prism travels…

Minimum deviation is the symmetric passage: equal angles at both faces, ray parallel to the base.

A glass slab produces no NET deviation because…

A slab only shifts the ray sideways (lateral displacement). A prism's faces are NOT parallel — that's the whole trick.

Deviation by a prism increases when…

δ depends on A, μ (and i) — not on brightness or physical size.

• parallel to the base, with i = e — correct
• along the base
• perpendicular to the base
• its two faces are parallel — the second bend undoes the first — correct
• glass is too thin to bend light
• slabs don't refract
• the prism angle A or the refractive index μ increases — correct
• the light is made brighter
• the prism is made larger overall