Prism Dispersion Lab

A prism does not add colour to light — it separates colours that were already there. White light is a mixture of every visible wavelength, and a prism sends each one on a slightly different path.

Why the colours separate

Light slows down inside glass. How much it slows depends on the wavelength: short wavelengths (blue) slow more than long ones (red). A bigger slowdown means a bigger bend at each surface. Run that bend twice — once entering, once leaving — and a tiny difference becomes a visible fan of colour.

In physics terms, the refractive index n depends on wavelength. This is called dispersion, and it is why “the refractive index of glass” is always quoted for one specific colour (usually the yellow sodium line, 589 nm).

• Red light (~650 nm): n ≈ 1.51 — bends least
• Blue light (~450 nm): n ≈ 1.53 — bends most

Two hundredths of a difference is enough to spread the beam across a wall.

The angle matters

At minimum deviation the ray passes symmetrically through the prism, and the total bending is at its smallest. This is the position physicists use to measure refractive index precisely, because the maths is cleanest there: n = sin((A + D)/2) / sin(A/2), where A is the prism angle and D the deviation.

Try this

• Rotate the prism slowly — the spectrum swings one way, pauses, then swings back. That pause is minimum deviation.
• Switch to a strongly dispersive glass: the fan opens wider.
• Set the prism angle to 0 (a flat slab) and the colours recombine into white on exit.

Why it matters

Dispersion is a nuisance in camera lenses — it causes the coloured fringes known as chromatic aberration, which designers cancel by pairing different glasses. It is also the working principle of a spectrometer, and the same physics paints a rainbow: each droplet acts as a tiny prism plus a mirror.