What is Wavelength, the identity card of a radiation? — Physics

Definition 2.3 High School Physics · Chapter 2 — Light Spectra and the Message of Light

Each monochromatic radiation is characterized by one number, its wavelength $\lambda$ , measured in nanometers ( $1\,\mathrm{nm} = 10^{-9}\,\mathrm{m}$ ). The wavelength is the identity card of the radiation: give $\lambda$ and you have said everything there is to say about the pure color. The eye responds to wavelengths between about $400\,\mathrm{nm}$ (violet) and $800\,\mathrm{nm}$ (red) — the visible spectrum. Beyond the two ends the radiation continues, invisible to us: ultraviolet below $400\,\mathrm{nm}$ , infrared above $800\,\mathrm{nm}$ . What the wavelength actually measures — the length of one ripple of a light wave — is explained in Chapter 8; in this chapter, the number itself is what matters.

Examples

Example 2.4 (Reading wavelengths)

A green laser pointer emits at $532\,\mathrm{nm}$ : inside the visible range, in the green. A germicidal mercury lamp emits at $254\,\mathrm{nm}$ : ultraviolet, invisible (and dangerous to the eye precisely because invisible). The diode of a television remote emits at $940\,\mathrm{nm}$ : infrared — point it at a phone camera, whose sensor sees a little beyond $800\,\mathrm{nm}$ , and the invisible flashes appear on the screen.

Example 2.17 (A flask of sodium vapor)

White light crosses a flask containing cool sodium vapor. The transmitted spectrum is a full rainbow with one dark line at $589\,\mathrm{nm}$ — precisely where a sodium lamp shines brightest. Looking at the flask from the side in a dark room, one sees the stolen light re-emitted: a faint yellow-orange gleam at that same wavelength.

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Examples

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