A cemented doublet is to be achromatised for the F and C lines from BK7 ( , , ) and…
PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Geometrical optics and imaging · Geometrical optics and imaging
What an Instrument Can See, Ch Aberrations: the phase written on the pupil
Problem
A 200mm cemented doublet is to be achromatised for the F and C lines from BK7 (nF=1.522376, nd=1.516800, nC=1.514322) and the flint F2 (nF=1.632081, nd=1.620040, nC=1.615035). Treat both elements as thin lenses in contact, so that their powers add.
((a)) Find the Abbe number of each glass.
((b)) Find the focal lengths of the crown and flint elements at the d line.
((c)) Treating each element's power as Φλ=K(nλ−1), with K fixed by its d-line power, find the doublet's secondary spectrum: how far the d line focuses from the common F and C focus, in micrometres.