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focal point

Each (rotation symmetric) lens has two focal points on it’s optical axis.
They’re located where images of infinite distanct objects are generated.
The focal points belong to the Gauss-points.

When a ray of light is sent parallel to the optical axis into a lens or lens system, then the ray or it’s prolongation intersects the optical axis after exiting the last lens.

This intersection with the optical axis is called focal point.

The name is derived from “burning glasses” (imagine a magnifying glass) with which the (nearly parallel) sun beams are bundled to one point.
At this point it gets so hot that wood or paper placed at this spot starts to burn.

Fraunhofer-Line

image of Frauhofer-Lines

Fraunhofer lines, (C) Wikipedia

Spectral absorption lines of the spectrum of the sun, named after the German physicist Joseph von Fraunhofer (1787–1826)
Such Lines are produced whenever a cold gas is between a broad spectrum photon source and the detector.
Photons are absorbed and then re-emitted in random new directions. Therefore the intensity in the original direction is lower and a dark line appears in the spectrum.

[table]
wavelength in nm, Fraunhopfer line, Lightsource, color
365.0146,i,Hg,UV
404.6561,h,Hg,violet
435.8343, g, Hg, blue
479.9914, F’, Cd, blue
486.1327, F, H, blue
546.0740, e, Hg, green
587.5618, d, He, yellow
589.2938, D, Na, yellow
643.8469, C’, Cd, red
656.2725, C, H, red
706.5188, r, He, red
768.2, A’, K, red
852.11, s, Cs, IR
1013.98, t, Hg, IR
[/table]
See also Abbe-number, dispersion.