The Deviation Dial

Why it matters

Send red and violet into the same prism at the same angle. They leave by DIFFERENT doors. Glass plays favourites — and the favouritism paints rainbows.

The idea in plain words

One medium, many speeds. Tap the terms.

μ(violet) > μ(red) → δ(violet) > δ(red)

Same prism: red deviates 38.2°, violet 39.6°. What is the angular width of the spectrum?

1.4° — small, but enough to unpack white light

• the cause: μ by colour — the refractive index varies with wavelength — 'dispersion' in one fact
• degrees: δ — deviation — how far the prism turns the ray; more μ, more turn
• the control test: in vacuum — ALL colours travel at c — dispersion only happens inside a medium
• width = δv − δr = 39.6 − 38.2

Predict first

Through the same prism, which colour deviates MORE?

μ(violet) > μ(red) in glass: shorter wavelengths travel slower inside, so violet bends hardest — every time, in every prism.

• Violet — glass slows it more (bigger μ) — correct
• Red — it has more energy
• Both the same — same prism, same angle

What you do

Fire red, then violet, at the same prism. Read δ for each — the gap between them is the seed of every rainbow.

Check yourself

Inside glass, which colour travels FASTEST?

μ = c/v: small μ means small slowdown. Red cruises; violet wades.

Deviation of a given colour by a prism does NOT depend on…

δ is set by A, μ and the incidence angle. Intensity changes how MUCH light, not where it goes.

A prism separates colours; a rectangular glass slab doesn't visibly, because…

Dispersion happens at the first face of both — but the slab's second face undoes the angular spread.

• Red — smallest μ, least slowed — correct
• Violet
• All equal, as in vacuum
• the brightness of the light — correct
• the prism angle A
• the material's μ
• the slab's parallel faces recombine the directions — colours exit parallel, only slightly shifted — correct
• slabs don't disperse at all
• slabs are thinner