CHM 320 Lecture 29 Chapt. 18Chapter 18 Fundamentals of SpectrophotometryHomework: Due Wednesday, April 19Problems – 18-1, 18-2, 18-4, 18-8, 18-10, 18-12, 18-16, 18-22CHM 320 Lecture 29 Chapt. 18The nature of light1. Light as a wave: light is described as a periodically oscillating electric and magnetic field, with perpendicular electric and magnetic field components 2. Light as a particle: light energy is transmitted as discrete “quanta” or packets called “photons”CHM 320 Lecture 29 Chapt. 18• Plane-polarized electromagnetic radiation of wavelength λ, propagating along the x axis. • The electric field of the plane-polarized light is confined to a single plane.• Ordinary, unpolarized light has electric field components in all planes.CHM 320 Lecture 29 Chapt. 18Energy of light can be described by its wavelength (λ, nm), wavenumber (ύ, cm-1), and frequency (ν, sec-1)Energy => E = hν = hc/λ = hcύin vacuum, velocity is independent of frequency, maximum valuespeed of light => c = νλ = 2.998 X 108m/sCHM 320 Lecture 29 Chapt. 18Each region of the electromagnetic spectrum serves as the basis for one or more important spectrometric techniques in chemical analysis.rotationvibratione- transe- transbond breakCHM 320 Lecture 29 Chapt. 18Colors of Visible LightWavelength Absorbed Observed380-420 violet green-yellow420-440 violet-blue yellow440-470 blue orange470-500 blue-green red500-520 green purple520-550 yellow-green violet550-580 yellow violet-blue580-620 orange blue620-680 red blue-green680-780 purple greenCHM 320 Lecture 29 Chapt. 18Nine properties of electromagnetic radiation• Diffraction• Refraction• Dispersion• Reflection• Scattering• Polarization• Transmission• Absorption• EmissionCHM 320 Lecture 29 Chapt. 18Diffraction refers to the constructive and destructive interference pattern that is formed when light passes through an opening of size d which is ~ same order as λ,CHM 320 Lecture 29 Chapt. 18Refraction is the change of direction which occurs when light passes from one medium to another.CHM 320 Lecture 29 Chapt. 18Dispersion refers to the apparent “spreading out” in distance or angle when light is diffracted or refracted.CHM 320 Lecture 29 Chapt. 18• Reflection is obvious, but is importantly exploited in optical fibers which work based upon “total internal reflection”CHM 320 Lecture 29 Chapt. 18Two types of scattering are of great importance: an “elastic” Rayleigh and an inelastic “Raman” varietyCHM 320 Lecture 29 Chapt. 18Polarized light is used in observing the properties of optically active compounds. Plane polarized and unpolarized lightCHM 320 Lecture 29 Chapt. 18Transmission refers to when light passes through a medium without a net change.CHM 320 Lecture 29 Chapt. 18Absorption refers to when the energy of the EMR is transferred to atoms or molecules of the absorbing medium, which subsequently wind up in higher energy statesCHM 320 Lecture 29 Chapt. 18Emission is the opposite of absorption: an atom or molecule in an excited state gives off a photon and returns to a lower-lying energy stateCHM 320 Lecture 29 Chapt. 18Absorption, emission, fluorescence• Absorption: atom or molecule “absorbs” a photon of specific energy, goes to a higher energy state• Emission: atom or molecule releases a photon of specific energy, goes to a lower energy state; nonradiative excitation• Fluorescence: absorption followed by re-emissionCHM 320 Lecture 29 Chapt. 18Ground State: The state of least possible energy in a physical system, as of elementary particles. Also called ground level.Excited State: Being at an energy level higher than the ground state.EnergyAbsorption