Measurements in Fluid Mechanics 058:180:001 (ME:5180:0001) Time & Location: 2:30P - 3:20P MWF 218 MLH Office Hours: 4:00P – 5:00P MWF 223B-5 HL Instructor: Lichuan Gui

2 Lecture 7. Optical experimentation: Nature of light

3 Background for optical experimentation Light - radiation that propagates through vacuum in free space, - in the form of electromagnetic waves, - wavelength - both oscillating transversely to the propagation direction - and normal to each other. ( classical Electromagnetic theory ) - intensities of the electric and magnetic fields oscillate harmonically in time t and along propagation direction x. T – period of oscillation - frequency of oscillation: =1/T - wave number:  =1/ - phase speed:  = /T = - speed of light propagation in vacuum: c =  10 8  300,000 km/s - relation between amplitudes of electric and magnetic fields:

4 Background for optical experimentation Wave front - a surface with constant phase in electric/magnetic filed. - plane wave: all wave fronts are plane - spherical wave - cylindrical wave

5 Background for optical experimentation - associated with the orientation of the plane of oscillation of the electric field. - randomly polarized (unpolarized) - circularly polarized - elliptically polarized - plane/linear polarized Polarization

Background for optical experimentation The colors of light Visible light: wavelength range nm Different types of radiationVisible light colors Refractive index c – light speed in vacuum v – light speed in medium

7 Background for optical experimentation Lorentz-Lorenz (or Clausius-Mosotti) express: Relationship between refractive index and density Gladstone-Dale formula- Simplified for gases e – charge of an electron m e – mass of an electron L – Loschmidt’s number m – molecular weight – frequency of visualizing light i – resonant frequency of distorted electron f i – oscillator strength of distorted electron n – refractive index K – Gladstone-Dale constant  – density In gas mixture of N components: Dependency of refractive index of water on temperature T c (20-34  C) for =632.8 nm:

8 Background for optical experimentation Light refraction Law of refraction Application of refraction: convergent and divergent glass lenses

9 Background for optical experimentation Light reflection - glass-air interface:  c =42  Law of reflection Critical angle Total internal reflection - glass-waster interface:  c =62 

10 Background for optical experimentation Light absorption Beer’s law: I – radiant intensity of passing light I 0 – radiant intensity of incident light  – absorption (attenuation) coefficient l – length of path Penetration Depth: - a measure of how deep light can penetrate into a material. - defined at which I=37%I 0 - small for transparent material - extremely large for opaque material Birefringence (double refraction) decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. - unequal indices of refraction in two directions

11 Homework - Questions and Problems: 1 and 2 on page Read textbook on page Due on 09/09