Light sources any object that is heated emits

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Light Sources Any object that is heated emits radiation . Emission from real objects such as a tungsten filament light bulb emulate blackbody radiation (the emission is a continuous spectral distribution). Visible and infrared lamps as light sources approach blackbody radiators. The radiation from an object's surface expressed as power per unit area is the exitance (emittance), M. M = σ T 4 Where σ is the Stefan-Boltzmann constant ~ 5.7 X 10 -8 W/(m 2 K 4 ) T = Temperature (K)
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Spectral distribution of blackbody radiation Lamps for absorption spectrometers Lamps are broad-band, polychromatic light sources. Typically they are inexpensive and stable. i) Visible and Near Infrared: Tungsten Lamp ii) Ultraviolet and Visible: Quartz Halogen Lamp iii) Ultraviolet: Deuterium Arc Lamp iv) Infrared: nichrome wire Spectral Distribution of Different Light Sources Lasers provide ~ single λ Very bright sources for spectroscopy Properties of Lasers: (i) Monochromatic (only one wavelength) (ii) Collimated (emit in one direction) (iii) Polarized (only one electric field vector)
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(iv) Coherent (electric/magnetic fields in phase) (v) expensive (vi) high maintenance Laser operation principles Operation of a Laser Population Inversion: A higher energy state has a greater population of electrons than a lower energy state Lifetimes of electrons in various energy levels: E 2 >> E 3 or E 1 Two popular lasers: helium-neon laser ~ red light Laser diode ~ solid state, p-n junction ~ red, or near IR Wavelength Selection Devices I) Prisms were used in older instruments, Quartz or salt crystals. II) Monochromators separate wavelengths of light; they consist of both entrance & exit slits, mirrors, and di ff raction grating or refraction lens/ prisms, and filters. Grating–monochromators Polychromatic light is collimated (focused) into a beam of parallel rays by a concave mirror (monochromatic-one wavelength; polychromatic-many wavelengths).
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Rays strike the reflection grating (see next figure) and di ff erent wavelengths are di ff racted (separated) at di ff erent angles. Di ff racted light is focussed by a second concave mirror so that only one wavelength passes through the exit slit at a time. Grating Equation: n λ =d(sin θ – sin φ ) n = di ff raction order (1…n) ; d = groove spacing; φ = angle of reflection; θ = angle of incidence Components of a Grating Spectrophotometer i) di ff raction grating A di ff raction grating is ruled with a series of closely spaced parallel grooves separated by distance d. These are often constructed from aluminum metal and coated with a non oxidative coating applied.
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When light is reflected from the grating, each groove behaves as a source of light. When adjacent rays are in phase, they reinforce each other. When adjacent rays are out of phase, the partially or completely cancel each other. Thus can be aligned to allow only certain wavelengths to pass through.
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