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Scattering

Scattering. Q1. The sky appears blue as a result of:. directional scattering of light that shows a strong inverse relationship with frequency non-directional scattering of light that shows a strong inverse relationship with frequency

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Scattering

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  1. Scattering

  2. Q1. The sky appears blue as a result of: • directional scattering of light that shows a strong inverse relationship with frequency • non-directional scattering of light that shows a strong inverse relationship with frequency • directional scattering of light that varies exponentially with frequency • non-directional scattering of light that varies exponentially with frequency

  3. Q2. The colors of sunset in a non-polluted atmosphere result from the fact that: • longer wavelengths scatter more in the late afternoon • shorter wavelengths have been depleted by their long passage through the atmosphere • longer wavelengths have been depleted by their long passage through the atmosphere • chromatic aberration focuses longer wavelengths at a greater distance from the source

  4. Retinal Photocoagulation and Chromophores Chromophores are molecules that absorb laser energy (photons) • Shorter wavelengths: • have higher photon energy and are more efficient at heating target chromophores • do not penetrate as far through tissue • elicit greater Rayleigh scatter • Longer wavelengths: • are less efficient at heating target chromophores • penetrate tissue to greater depths and may damage underlying tissue • elicit less Rayleigh scatter

  5. Larger particle (Mie) Scattering Page 3.98 Scattering for particles larger than the wavelength of light quickly becomes directional due to lack of coherence of “interfering” waves with increasing angle of scatter Scattering is strongest in the forward direction and is weakly wavelength dependent

  6. Mie Scattering and Interference Low spatial coherence - more waves out of phase  much lower intensity Most waves in phase  constructive interference

  7. Rayleigh scatter Mie scatter Rayleigh and Mie Scattering Page 3.98 Non-directional (a) Fig 3.87 Directional (b)

  8. Mie and Rayleigh Scattering Blue sky  Rayleigh scatterStrong  dependence White clouds  Mie scatter (weak  dependence)

  9. Mie Scattering - Fog

  10. Mie Scattering - The Tyndall Effect A beam of sunlight is clearly visible in a dusty room due to Mie scatter. This is called the Tyndall Effect

  11. Tyndall Effect

  12. Tyndall Effect

  13. Mie Scattering - The Tyndall Effect A beam of sunlight is clearly visible in a dusty room due to Mie scatter. This is called the Tyndall Effect • Inflammatory exudate produced by cells floating in the aqueous (aqueous flare) can be seen with a broad slit-lamp beam for the same reason

  14. Mild aqueous flare http://www.monoptica.com/monoptica/students/gallery/ant_eye.asp

  15. More prominent aqueous flare Pseudo-hypopyon Uveitis due to Acute Lymphoblastic Leukemia

  16. Summary: Scattering • Rayleigh (molecular level) Scatter: • strongly wavelength-dependent • non-directional (random dipole scatter) • explains reduced penetration depth of short-wavelength ophthalmic lasers Mie (larger particle) Scatter: • weak wavelength-dependence • strongly directional • used to detect aqueous flare (inflammatory exudate in AC)

  17. Intraocular Light Scatter Page 3.99 • (Disability) Glare: contrast-lowering effect of stray light in a visual scene. • Sensitivity to glare: amplified as scattering in the cornea or crystalline lens increases. • Intraocular Light Scatter (Normal Eyes) • Cornea 30 % • Lens 70 % • Aqueous & vitreous < 1 %

  18. Glare and Contrast Page 3.99 • Scattered light: • raises the luminance of both target and background to the same extent  reduces contrast • forms a luminance “veil” which reduces the visibility of the target

  19. Glare and Contrast Page 3.99 • Scattered light: • raises the luminance of both target and background to the same extent  reduces contrast • forms a luminance “veil” which reduces the visibility of the target • Glare and glare sensitivity are increased in anterior segment diseases and cataract due to light scatter • Retinal disease does not produce light scatter and thus does not cause glare

  20. Sources of Disability Glare Page 3.99 Age: I/O light scatter increase rapidly in middle age. Causes: (a) increasing lens fluorescence (converts incident UV light into scattered blue light) (b) lens yellowing (c) senile miosis: reduces retinal illumination and increases scattering from the edge of the pupil (d) subclinical lens opacities Cataract: markedly increases glare. Disproportionately greater glare in posterior subcapsular cataract Other: posterior chamber IOLs, posterior capsule opacification after cataract removal, keratoconus, corneal edema, Radial Keratotomy (RK), vitreous opacities

  21. Q3. An optical system has been corrected for all, but one, monochromatic aberration. The system consists of a negative spherical lens and an aperture stop to the right of the lens. For a monochromatic plane object, this system will produce : • Curvature of field • Pincushion Distortion • Barrel Distortion • Transverse chromatic aberration

  22. Next Chapter 7: The Pupil Chapter 8: Crystalline Lens

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