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Instrumental Lecture 7

# Instrumental Lecture 7 - Reflection = EM strikes a boundary...

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Reflection = EM strikes a boundary between two media differing in η and bounces back Specular reflection = situation where angle of incidence ( θ i ) equals angle of reflection ( θ r ) Medium 1 (air) η = 1 .00 Medium 2 (glass) η = 1 .50 Incident ray θ 1 θ 2 Reflected ray

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I r ( η 2 - η 1 ) 2 Reflectance = R = ---- = -------------- I i ( η 2 + η 1 ) 2 Where I i and I r = incident & reflected intensity For radiation going from air ( η = 1 .00) to glass ( η = 1 .50) as shown in previous slide R = 0.04 = 4 % Many surfaces at 4 % each (i.e., many lenses) can cause serious light losses in a spectrometer. This generates stray radiation or stray light .
Reflectance varies with the angle of incidence Percent Reflectance (R) Angle of Incidence ( θ i ) For monochromatic radiation, as incident angle deviates from the normal, the R tends to increase EM going from air ( η = 1 .00) to glass ( η = 1 .50) R = 4 %

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EM going from glass ( η = 1 .50) to air ( η = 1 .00) Percent Reflectance (R) Angle of Incidence ( θ i ) EM going from a medium of higher η to a medium of lower η , the angle of incidence can only increase to the critical angle before all of the light is completely reflected back R = 4 % Critical Angle
Critical Angle ( Ф C ) Normal to surface Medium 1 (air) Medium 2 (glass) Incident ray is 90 o to normal this is called grazing incidence Ф 1 Ф 2 Refracted ray Maximum value of Ф 2 Ф 2 = Ф C At 90 o incidence sin Ф 1 = 1 .0 η 1 sin Ф C = ----- η 2 Important for: 1 ) Refractometers 2) Fiber Optics

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Critical Angle ( Ф C ) Normal to surface Medium 2 (air) Medium 1 (glass) Ф 2 Ф 1 Incident ray Maximum value of Ф 1 for Refraction Ф 1 = Ф C When incidence is at the Critical angle, refraction is at 90 o η 2 sin Ф C = ----- η 1 Important for: 1 ) Refractometers 2) Fiber Optics Refracted ray
Critical Angle ( Ф C ) Normal to surface Medium 2 (air) Medium 1 (glass) Ф 2 Ф 1 Incident ray New value of Ф 1 for Reflection Ф 1 > Ф C At angles greater than the Critical angle, 1 00 % reflection occurs or Total Internal Reflectance Important for: 1 ) Refractometers 2) Fiber Optics Refracted ray Reflected ray

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Applying UV-vis Spectrophotometry Qualitative (as discussed) I) The UV-vis absorption spectrum is related to molecular structure, therefore it provides qualitative information as to structure both from ε and λ . In order to take advantage of this it is desirable to have a scanning instrument to measure the entire spectrum (either a double beam, direct reading instrument or a single beam unit with computer interface. By itself an electronic spectrum is not sufficient to identify a compound (while an IR spectrum is)
Quantitative II) The UV-vis region of the spectrum is particularly well suited to quantitative analysis for several reasons (both chemical & spectroscopic reasons): 1 ) Large values for ε make it possible to measure low concentrations 2)

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Instrumental Lecture 7 - Reflection = EM strikes a boundary...

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