METR Advanced Atmospheric Radiation Dave Turner Lecture 4

Reflection and refraction ΘtΘt Θ N1N2N1N2 Θ relative index of refraction Θ angle of incidence Θ t angle of refraction of the transmitted wave Discontinuity in N: EM wave partially reflected and transmitted Refraction means change of propagation direction of the transmitted wave through change of phase velocity partial reflection N 1 = 1 (air) N 2 = 1.33 (water)

Refraction Fig. 4.3 Petty (2006) Snell’s law  ray is refracted towards the optically thicker medium follows from Maxwell’s equations for a plane wave with suitable constraints on magnetic and electric fields at the boundaries N 1 = 1 (air) N 2 = 1.33 (water) Snell’s law

Total reflection n 2 <n 1 Refraction away from the normal  critical angle for total reflection (Θ 2 =90°) Θ > Θ C total internal reflection within the medium Θ C also maximum transmitted angle at an incident angle of 90° Wikipedia N 1 = 1 (air) N 2 = 1.33 (water) Θ C = 49° Mirages are explained by this

Reflection Fig. 4.3 Petty (2006) N 1 = 1 (air) N 2 = 1.33 (water) specular reflection: ray is reflected like elastic ball thrown at floor angle of incidence = angle of reflection surface irregularities much smaller than wavelength (homogeneous surface) inhomogeneous surfaces

8 Rainbow 2cnd rainbow due to double reflection 8

GPS Radio Occultation At radio wavelengths (lambda ~ 1 m), the refractive index is function of temperature and humidity

Forward Single Scattering Probe (FSSP)

HeNe laser illuminates particles passing through center of probe “Dump spot” removes any signal within 4° of laser path through instrument Detector is sensitive to intensity of scattered light within 12° of laser path Integrated signal is function of size of particle (assume water spheres to convert)

Log10 (Signal) [arbitrary units]

Liquid water with m = – 0.982i