Cornu Mounting
Definition and meaning of Cornu Mounting in chemistry.
Cornu mounting is a compound quartz-prism design used in ultraviolet spectrographs, built from a right-handed and a left-handed quartz prism cemented together with their optic axes aligned along the light path so that their equal and opposite optical rotations cancel.
In more detail
Ordinary glass absorbs strongly below about 350 nm, so early UV spectrographs used crystalline quartz, which stays transparent down to roughly 185 nm. Quartz is optically active and birefringent for light traveling off its optic axis, which would otherwise rotate the plane of polarization and split the beam into two differently refracted images. By threading the light along the optic axis of each half-prism, the Cornu design avoids this double refraction, while the opposite handedness of the two quartz halves makes their rotary powers offset one another, so the emergent beam disperses cleanly like light from an ordinary single dispersing prism.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Also known as | Cornu prism, Cornu-type quartz prism |
| Construction | right-handed + left-handed quartz prism halves, optic axes along the beam |
| Named after | Marie Alfred Cornu, French physicist (1841-1902) |
A Cornu quartz prism served as the dispersing element in early quartz-prism spectrographs used to record ultraviolet emission and absorption spectra for elemental and molecular analysis.
Frequently asked questions
Why use quartz instead of glass for these prisms?
Ordinary glass strongly absorbs ultraviolet light below about 350 nm, while crystalline quartz remains transparent down to about 185 nm, making it necessary for prism spectrographs that need to disperse UV spectra.
What specific problem does the Cornu mounting solve?
A single quartz prism used off its optic axis is birefringent and optically active, splitting and rotating the transmitted light. Cementing a right-handed and left-handed quartz prism together, with the optic axis along the beam, cancels the net optical rotation while avoiding double refraction.