CHEM 524 Course Outline (Sect

CHEM 524 Course Outline (Sect. 4) - 2005

FOR A PDF VERSION OF THIS SET OF NOTESWITH EMBEDDED FIGURES CLICK HERE (this may not be available for download until Sept.)

III. Optics — Control of light — move it from source to detector through experiment

A. Lenses + Mirror (Text: Ch 3 & 1,4) design — shape & materials — efficiency

1.Basic concepts: index of refraction — n = c/v, c = 3 x 10⁸m/s,

n_glass ~ 1.5, n_CaF2 ~ 1.35, n_ZnSe ~ 2.5, n_Ge ~ 4 – liquids as well: n_water ~ 1.33 , n_alcohol ~ 1.36, n_CCl4 ~ 1.466, n_{Br-napthalene} ~ 1.659

non-isotropic depends on direction: quartz: n_o ~ 1.544 n_e ~ 1.553, zircon: n_o ~ 1.923 n_e ~ 1.968 (uniaxial crystals, o=ordinary, xx and yy, e=extraordinary, zz)

conservation law: r(l) + a(l) + T(l) = 1 - mirror T~ 0 & lens T ~ 1

dispersion (index, n, increase with dec l) — dn(l)/dl < 0 (also dispersion generally dec. with inc. l)

(exception, if absorption band, index is complex and has singularity, derivative shape)

Snell’s law of refraction: n, sin q₁ = n₂ sin q₂,

reflection: q₁ = q₃ vs. refraction: q₂ < q₁ for n₁ < n₂

reflection loss: r(l) = (n₂-n₁/n₂+n₁)², normal incidence, e.g. air/glass ~ 4%, but ZnSe ~18% per surface

Brewster angle, — zero reflection loss in one polarization (II to reflection plane) at specific angle q_B = tan^-1(n₂/n₁)

Total internal reflection, - n₁>n₂, r(l) max at q₁=q_c, q_c = sin^-1(n₂/n₁)

air — glass, q_c ~ 42^o, useful property for prism reflectors (no coating, higher power possible)

2. Mirrors: spherical mirror imaging — reflection,

materials — Al(uv), Ag (vis), Au(IR), coating can help r -- VUV—MgF2, vis-SiO

spherical mirror focusing: mirror formula: 2/R = 1/ S₁ +1/ S₂ =1/f,

S₁ — object — O, S₂ — image — I, R — radius (R<0 concave), f — focus

S₁= infinite, parallel beam,à S₂ = f, S₁ = S₂ = -R à 1:1 imaging

magnification: m = -I/0 = - S₂/ S₁

result: S₁ > R — demagnify, f < S₁ < R — magnify, S₁ < f — no image

variations: plane, convex (virtual positive image), aspherical, elliptical, parabolic, off-axis parabola

Prism—use total internal reflection, no coating, high power

3. Lens: refraction straight line design, must transmit but losses due reflection or absorption

spectral region, material dependent:

quartz — uv (180nm) to near IR ~ 3 m. — 4.5 m.
CaF₂ — vuv (140nm) to mid IR ~ 8 m.
ZnSe — yellow (~500 nm) to IR ~ 16 m.
Ge — near IR (~2 m) to IR ~ 20 m.

Thin-lens

operative formula: 1/ S₁ +1/ S₂ =1/f,

lens makers formula for one surface: n₁/ S₁ +n₂/ S₂ = (n₂ -n₁)/R,

typically purchase based on size and focal length so in practice ignore radius, but can increase efficiency by choosing best combination

magnification: same as mirror: m = -S₂/S₁

special designs: cylindrical (focus one dimension), aspherical (reduce aberration)

AR coating — reduce reflection loss (n — index lens, n Å 1 air) - r=(n-1/n+1)²

-- add l/4 layer of intermediate index n₁ , or multiple layers— goal: zero reflection by interference, sensitive to angle

-- multilayer (N)— get zeros at (N — 1) l’s

4. Light gather power — trade off: more light or smaller image (m = 1 often best):

closer to source, collect more BUT image bigger (further, brighter image, smaller spot BUT less light)

F-number: [F/n] = f/D, if not circular shape: D = (4A/p)¹^/2

Called the speed — smaller is faster

Irradiance (goes as square): halve F/n, quadruple light (e.g. camera people: F/1.4 is twice the light of F/2.0)

Varies as solid angle, W, E=B_s(p/4)/(F/n)²

5. Aberrations (solution): chromatic (compound lens, mirror), spherical (reduce aperture, plano-convex), coma (align, red. apert.), astigmatism (reduce off-axis mirror, parabolic)

B. Special

1. Fiber optics — total internal reflection — limits acceptance angle

2. Beam Splitter — divides beam in space (can be coating or just surface, can use angle to enhance, single surface best)

3. Filters — color filters are glasses with absorbing materials mixed in, ex. Salts of transition metals, band pass or cut off, progressing out to even IR

--interference — multiple layers of dielectric act as Fabry-Perot interferometer, narrow -band or cut off:

Homework—read Chap 3-1, 2, 3, 4, 5 (will carry over to Section 5, Special Optics)

Problems to do: Ch 3: 2, 7, 12, 18, 26

Links to optics etc:

Melles Griot Optics tutorial (also sell optics)

http://www.mellesgriot.com/products/optics/opticaltutorial.asp

Newport-Oriel Optics section (also sell optics):

http://www.newport.com/Optics/1/productmain.aspx

Iowa State course, properties of light (sort of just formulas),

http://avogadro.chem.iastate.edu/CHEM513/513-1.pdf

physicaloptics

http://avogadro.chem.iastate.edu/CHEM513/513-2.pdf