Experiment #45 · Scientific experiment

Newton's Prism Experiment

White light is composite

Isaac Newton · 1672 · Optics

First published: I. Newton, "New Theory about Light and Colors", *Phil. Trans. Roy. Soc.* 6 (1672): 3075–3087.

A glass prism splits white sunlight into a spectrum — and a second prism, properly placed, recombines it. White light is not pure; it is a mixture of all colours.

In a darkened room, Newton passed a narrow beam of sunlight through a glass prism and observed the familiar spectrum of colours projected on the opposite wall. The accepted view (Aristotelian and Cartesian) was that colour arose from modifications of pure white light by the prism. Newton's decisive *experimentum crucis*: select one colour from the spectrum, pass it through a second prism, and observe that the light is no longer refracted into other colours — it retains its colour and refractive angle. White light is therefore composite, made up of pre-existing constituents with distinct refractive indices. The prism does not modify; it sorts. The case is a foundational example of how a clean experimental design can dispatch a tradition-bound theory.

Formulation

Sunlight → narrow aperture → prism → spectrum on wall. Aristotelian prediction: prism modifies white light into colours. Newton's *experimentum crucis*: spectrum → second aperture isolating one colour → second prism. Predicted (if modification): further dispersion. Observed: no further dispersion; pure colour retains identity. Conclusion: white light = mixture of pure colours.

Compare this experiment

Dimensions Engaged

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Matter

Bears on Matter · Ontological Status: light is composite, with parts (different "rays") existing prior to separation. A materialist reading of optics.

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Energy

Engages Energy · Conservation: each spectral component is conserved through the prism; the prism does not create or destroy.

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Observer

Indirectly: opens the long debate over primary vs secondary qualities — is "redness" a property of the light, of the eye, or of the relation between them?

Responses — How Schools Engage

Affirms / takes the bait 4

Naturalism

A model case of crucial-experiment methodology: a single well-designed test settles a long-standing theoretical dispute. Newton's optics becomes the paradigm of mathematical physics.

Realism

Scientific realism: the spectral components are real constituents of white light, discovered (not constructed) by the prism. Galilean primary qualities extended to optics.

Empiricism

A canonical demonstration of empirical method: observation, controlled variation, decisive test. British empiricism took Newton as exemplar.

Structuralism

Light is structural: refractive index is a relational property linking wavelength to bending angle. The experiment exposes the relational nature of optical phenomena.

Reframes the question 2

Phenomenalism

The spectrum is real as a pattern of sensation; the inferred "constituents" of white light are theoretical posits useful for organising those sensations. Phenomenalists demur on the realist gloss.

Kantian Transcendental Idealism

Empirically the experiment is decisive; transcendentally it leaves untouched the question of what light is *in itself*. Newton's physics is phenomenally accurate without being metaphysically ultimate.