Lenses - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Lenses - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Lenses – lens equation (for a thin lens) 1 1 1 ---- = (η – η’) ------ - ------ f r1 r2 Where f = focal length η = refractive index of lens material η’ = refractive index of adjacent material r1 = radius of curvature of first surface r2 = radius of curvature of second surface 1 1 1 ---- = ---- - ---- f i o object image o f i distance to image distance to object docsity.com Focal length is important specification of a monochromator focal length (f) f/ (f number) = ------------------------------ lens clear aperature • f/ is measure of light gathering power • Larger f/ means getting less light • Light gathering power ~ 1/(f/)2 Point source at f (focal point or focal length) Parallel beams docsity.com Astigmatism – for an object off axis, the horizontal and vertical focuses differ – get two images displaced from each other Numerical Aperture (NA) = sin θ angle over which a device accepts light Slits – entrance and exit slits Slits affect energy throughput & resolution Decrease slit width
gain resolution & lose energy throughput Open slits wider
increase signal (throughput) but lose resolution θ docsity.com Energy throughput must be sufficient for detector to measure signal with adequate precision. In practice the image of the entrance slit in a monochromator should just fill the exit slit for optimum conditions. Otherwise the larger slit establishes (i.e, limits) the resolution and the smaller slit establishes (or limits) the energy throughput. There is a theoretical minimum for slit widths imposed by diffraction. docsity.com Light exiting a monochromator exit slit has a triangular distribution Optical Efficiency = throughput x resolution Good criterion for comparing optical systems Prism < Grating < Interferometer Monochromator Monochromator Relative power -20 -10 λo +10 +20 Range of λ’s passing when set at λo bandpass or bandwidth or spectral slit width docsity.com Dispersion Devices 1) Prisms dη dη ----- depends on material, ----- greatest at shorter λ dλ dλ b A θ A = apical angle b = base length η λ docsity.com mm dθ Linear Dispersion ------ = f ----- nm dλ Depends on angular dispersion and focal length For constant bandwidth, slit widths must be varied with λ to compensate for variations in dη/ dλ Stated another way, linear dispersion changes in different regions of the spectrum docsity.com Kinds of Prisms Littrow Prism & Mounting – compact design Focal Plane Reflecting Prism docsity.com Another view of a Cornu prism r--*+--=Optic axis Ik Red Green Blue UV ‘_ Right Oe Cornu prism of quartz. The circular double refraction (not shown) produced by the first half is just offset by the equal and opposite effect in the second half. Two overlapping spectra would result if the prism were all of one kind of crystalline quartz. ® docsity.com Gratings – based on diffraction & interference Transmission Gratings & Reflection Gratings consist of a series of grooves in glass or quartz or a mirror (usual kind) Monochromatic Radiation Grating d docsity.com Gratings work on the principles of diffraction & interference docsity.com