Experimentum Crucis

LITIn his 1672 letter to Oldenburg (Phil. Trans. 6, 3075) Newton isolated one colour from a prism's spectrum and sent it through a second prism: it bent again but did not change colour. Light, he argued, is a mixture of rays each with its own refrangibility. Here the source is a quicksilver lamp. Its visible lines are taken from the NIST Handbook of Basic Atomic Spectroscopic Data (air wavelengths): 404.656, 435.833, 546.073, 576.960 and 579.066 nm.

MODELRays are traced through two equilateral prisms with Snell's law. The glass is modern N-BK7 (Schott Sellmeier coefficients, n = 1.5168 at 587.6 nm); Newton's glass was different. Choose which line passes the slit; the second prism deviates it further but its colour stays one line. Move the slit and the spectrum shows its fine structure: the yellow pair is 2.1 nm apart.

◀ slitslit ▶open slit (all lines)

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