Refraction Through a Prism: 3 Clear Examples & How It Works

Refraction through a prism is what happens when light passes through a triangular block of glass and bends twice — once entering and once leaving — ending up in a direction different from where it started. The key difference from a flat slab is that the two surfaces aren't parallel, so the light emerges at an angle, not just shifted sideways. When that light is white, each colour bends by a different amount and the beam fans out into a rainbow spectrum. Here's the full physics — from Snell's law at both faces to the angle of minimum deviation.

What is refraction through a prism?

A prism is a transparent block — usually glass or acrylic — with two flat, polished surfaces that meet at an angle called the apex angle (or refracting angle), typically 60° for laboratory prisms. The third side is the base.

Refraction through a prism works like this:

1. Light enters the first face travelling from air (rarer medium) into glass (denser medium) — it slows and bends toward the normal
2. Inside the prism, light travels in a straight line, now angled relative to the base
3. Light exits the second face travelling from glass back into air — it speeds up and bends away from the normal

Because the two faces are slanted toward each other, the two bends add up instead of cancelling out, and the beam emerges in a new direction. The total angular change — the difference between the original and final direction — is the angle of deviation.

This is where a prism differs from a flat glass slab. In a slab the faces are parallel, so the ray enters and exits at the same angle and the net direction doesn't change — only a small sideways shift. In a prism, there's real angular deviation. (The Physics Classroom's dispersion tutorial walks through the geometry in more detail.)

How light travels through a prism: step by step

Here's the picture that makes it stick. Think of a runner entering a patch of mud at an angle — one foot hits the mud first and slows, so the runner pivots toward the mud side. Then imagine the runner leaving the mud patch through a second edge that's also angled — the foot that leaves first speeds up first, and the runner pivots the other way. That's exactly what light does entering and exiting a prism.

Let's label the angles. Call the apex angle of the prism α (typically 60°). When a ray hits the first face at an angle of incidence θ₁ (measured from the normal):

• It bends to an angle φ₁ inside the glass, given by Snell's law: n_air sin θ₁ = n_glass sin φ₁
• The ray then travels to the second face, where it meets that surface at an angle φ₂ measured from the normal inside
• It emerges into air at an angle θ₂, again following Snell's law: n_glass sin φ₂ = n_air sin θ₂

The geometry of the prism links the internal angles: φ₁ + φ₂ = α (the apex angle). The total deviation D is the sum of both bends: D = θ₁ + θ₂ − α.

For crown glass (n ≈ 1.52) with a 60° prism and a typical incident angle of 40°, the ray emerges at around 65° — a deviation of about 45° from its original direction.

Why a prism splits white light: dispersion

Here's the part that makes refraction through a prism so famous. White light isn't a single colour — it's a mixture of every wavelength from about 380 nm (violet) to 700 nm (red). And here's the crucial fact: glass has a slightly different refractive index for each wavelength.

For crown glass at 20 °C:

• Red light (660 nm): n ≈ 1.512
• Yellow light (580 nm): n ≈ 1.518
• Blue light (470 nm): n ≈ 1.524
• Violet light (410 nm): n ≈ 1.530

Violet slows the most and bends the most; red slows the least and bends the least. Inside the prism, the colours separate into a fan, and by the time they emerge, they're spread out into the familiar sequence: red, orange, yellow, green, blue, indigo, violet.

A common misconception worth correcting: people sometimes imagine the prism adds colour to white light. It doesn't. In the 1660s, Isaac Newton let sunlight through a prism, saw the spectrum, then passed just one colour through a second prism — it stayed that colour. He then recombined the colours back into white with a third prism. The colours were inside white light the whole time; the prism only separates them by how much each bends. (The Wikipedia article on optical prisms covers the different types of prisms and their uses in detail.)

The angle of minimum deviation

There's a special case that lab technicians and textbook writers love. If you rotate the prism while keeping the incident ray fixed, the deviation D changes. It reaches a minimum — the angle of minimum deviation, D_min — when the ray inside the prism travels parallel to the base. At this point, the angles on both faces are symmetric: θ₁ = θ₂ and φ₁ = φ₂.

The minimum deviation is important because it gives a very clean formula for the refractive index:

n = sin((D_min + α) / 2) / sin(α / 2)

This is how refractive indices are measured in optics labs: put a prism of the material on a spectrometer, find the angle of minimum deviation for a known wavelength, and plug it in. The method is accurate to several decimal places.

For a 60° crown glass prism, D_min is about 51° for yellow light, giving n ≈ 1.52 — right where it should be.

Glass slab vs prism: what's the difference?

This is why a window doesn't produce a rainbow but a prism does — the parallel surfaces of a window cancel the bend on entry and exit, while a prism's angled surfaces add them up.

Real-world objects that work like prisms

Refraction through a prism isn't limited to triangular glass blocks. Any transparent object with non-parallel surfaces produces the same effect:

• Raindrops — Sunlight entering a raindrop refracts, reflects off the back, and refracts again on exit, producing a full rainbow at about 42° from the antisolar point
• Diamonds — A diamond's high refractive index (2.417) and strong dispersion produce the brilliant flashes of colour known as "fire"
• Glass chandelier crystals — The angled cuts create tiny spectra that scatter coloured light around a room
• Ice crystals in the upper atmosphere — Hexagonal ice crystals act as prisms, producing 22° halos around the Sun or Moon
• Bevelled mirrors and glass edges — The angled cut on a mirror's edge can produce a thin spectrum

For more examples of light bending at boundaries, see our guide to refraction examples. To understand the deeper mechanism of why speed changes cause the bend, read what causes refraction. And for how water specifically bends light, see refraction of light in water.