SCATTERING & ABSORPTION OF LIGHT And APPLICATIONS
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➢ Deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media
➢ It is a consequence of the interaction of light with the electric field of scattering particle
➢ It is the primary mechanism of physical observation
Scattering of light occurs as follows:
➢ An incident photon induces oscillation of electron cloud of the particle which results in periodic separation of charge within the particle
➢ This separation of charge is called induced dipole moment
➢ The oscillation of this induced dipole is manifest as a source of electromagnetic radiation thereby resulting scattering of light
Radiation scattered from a particle depends on:
➢ Size of the particle
➢ Shape of the particle
➢ Index of refraction of particle
➢ Wavelength of radiation
Types of scattering
I. Elastic Scattering
II. Inelastic Scattering
Elastic scattering
➢ The energy of the incident photon is conserved
➢ Light scattered by the particle is emitted at the identical frequency of the incident light
➢ Types of elastic scattering:
▪ Rayleigh Scattering
▪ Mie Scattering
▪ Nonselective Scattering
➢ The energy of the incident photon is not conserved.
➢ Inelastic scattering includes:
▪ Brillouin scattering
▪ Raman scattering
▪ Inelastic X-ray scattering
▪ Compton scattering
Inelastic scattering
Rayleigh Scattering
➢ It occurs as a result of radiation being scattered by a particle which is smaller than the wavelength of the incident light
➢ It is very weak scattering & depends very strongly on wavelength
➢ Scattering produced by such small particles is isotropic i.e. equal in all direction
➢ Scattering efficiency(Kλ) is inversely proportional to the fourth power of the wavelength of light(λ)
i.e. Kλ α 1/ λ⁴
➢Nitrogen and oxygen in atmosphere are smaller than wavelength of UV and Visible light. So sunlight undergoes Rayleigh scattering in atmosphere
Why is the sky blue
o As sunlight moves through the atmosphere, longer wavelengths(eg.red) pass straight through
o However, shorter wavelengths(eg.blue) interact with gas molecules and scatter in the atmosphere
Secret of red sunset
o As the sun approaches the horizon during sunsets, sunlight travels longer distance to reach our eyes
o Hence, light with shorter wavelengths(eg.blue) are scattered more before reaching to our eyes and thus sunsets appear red
Mie Scattering
➢ It occurs when the size of the particle becomes equivalent to or greater than the wavelength of the incident light
➢ Scattering changes from being isotropic to a distortion in forward scattering direction
➢ White glare around the sun is also due to Mie scattering
➢ Cloud droplets being larger scatter all wavelengths of visible light. So the cloud appears white
Attenuation in optical fiber
o Involves scattering of light: due to change in local refractive index
o Also involves absorption of light: UV absorption, infrared absorption & ion resonance absorption
Nonselective Scattering
➢ Occurs when the particles are much larger than the wavelength of the radiation
➢ Caused by water droplets and large dust particles
➢ Also known as geometrical scattering
E.g. Rainbows
Scattering Process
Wavelength Dependence
Particle Size (in µm)
Kind of Particles
Rayleigh Scattering
λ^‾⁴ << 0.1 Air molecules
Mie Scattering λ^˚ to λ^‾⁴ 0.1 to 10 Smoke, cloud droplets
Nonselective Scattering
λ^˚ 10 Larger dust particles, water droplets, etc
Comparison
Ocular Scattering of light
➢ When light enters the eye, it is scattered as a result of optical imperfections in the eye(like various proteins, lipid particles, lamellar bodies, etc).
➢ This scattering can be sub-divided into:
a) Forward scatter: Light scattered toward the retina
b) Backward scatter: Light scattered backward
The scattering material interferes with vision in
two ways
i. Glare Effect: When a light from a source reaches the eye, a fraction of the light scattered within the ocular media falls on the retina. That light which falls in the foveal area lowers the contrast in the image of interest
ii. Light Reduction Effect: When the scattering is very strong, there occurs a reduction in the light available to form the image on the retina
Scattering of light occurs in various
pathological conditions:
❖Corneal haze in corneal edema
o Corneal edema: caused by excess water in the stroma;disrupts the very regular close-packed collagen structure of stroma; loss in corneal transparency
Corneal haze
Normal cornea
❖Age Related Nuclear Cataract
➢ Light scattering from micrometer sized particles surrounded by lipid shells: multilamellar bodies(MLBs)
➢ MLBs are the major source of forward light scattering:reduces contrast of fine details, particularly under dim light in ARNC
Due to ARNC
❖Flare In Anterior Chamber
➢ It is caused by scattering of light by the proteins in the aqueous humour
❖ Sclerotic Scatter Illumination
➢ It is an indirect illumination technique in slit lamp
➢ Light beam is focused mainly to the temporal sclera (mainly at the limbus)
➢ Total internal reflection occurs within the cornea. So the light pass through the substance of cornea and illuminate the opposite side of limbus
➢ If there is any pathology like corneal opacity, corneal scarring, etc it becomes visible as it scatters the ray of light
ABSORPTION OF LIGHT
➢ It is a process by which radiant energy is taken up internally by a substance or the medium through which it passes
➢ Light energy is transformed in to internal energy of the absorber such as thermal energy
Incident
Types of absorption
➢ Neutral Absorption: all wavelengths are equally absorbed
➢ Selective Absorption: some wavelengths are absorbed and others are transmitted; in colored glass, dyes, etc
❑A substance which absorbs all radiations is called a blackbody
Black Hole
➢ The amount of absorption mainly depends on:
a) the properties of the material
b) the thickness of the material
Absorption factor:
o It is the ratio of the absorbed luminous flux
to the incident luminous flux
❑Absorption is usually expressed in optical density(OD)
OD=log(1/T)
Where T=Transmittance
An OD of 1 represents transmittance of 10%
An OD of 2 represents transmittance of 1% and so on
Fluorescence
➢ It is a property by which substance absorbs light of a given wavelength and re-emits it as radiations of a longer wavelength
E.g. Fluorescein
Fluorescent imaging of three components in a dividing human cancer cell
Fluorescein
o It is a weak dibasic acid of molecular wt. of 330
o It is a yellowish-red compound which fluoresces a brilliant yellow-green under ultraviolet or blue illumination
Fundus photograph in FFA
Fluorescein spectrum
❖ Colors of Objects
o The color of an object is determined by the wavelengths of light that the object absorbs, transmits and reflects
Application of absorption of light
❖ Atmospheric Absorption of Radiations
o Ozone, water vapour, carbon dioxide, oxygen, nitrogen, etc present in the atmosphere absorb the specific wavelengths emitted from the sun
o Green house effect
❖ Photosynthetic absorption of light
o Chlorophylls absorb particular wavelengths of light and converts into chemical energy: basis of food cycle
❖ X-Ray
o X-rays are absorbed by different extends by different tissue,bone in particular, which is the basis for X-ray imaging
❖ Radiation Absorption By Ocular Tissues
o Tears and cornea: Far UV ( 180-315 nm)
Far IR ( 1400 nm- 1 mm)
o Aqueous humor absorbs very little radiation
o Lens: Near UV (315-390 nm)
IR > 2500 nm
UV absorption by lens increases with the increasing age
o Vitreous body: UV < 290 nm
IR > 1600 nm
Effects: Cataract, macular degeneration
❖ Absorption of light by photoreceptors
o The photochemical reactions occurred in the photoreceptors by the absorption of light forms the basis of the visual system
❖ Light Filters
o Material used to absorb or transmit light of all wavelength equally i.e. neutral density filter or selectively such as the colored filters
E.g. green filters, blue filters
❖ Absorptive Lenses
o Absorption may be uniform or selective
o Some lenses absorb mostly in the IR region of spectrum. E.g. Calobar, Ray Ban
o Other absorb in UV region. E.g. Spectacle Pink, UV 400, UV 530
o Colored contact lens,tinted lens
REFERENCE
•Optics by A. H. Tunnacliffe
•Optics and Refraction by A. K. Khurana
•Clinical Optics (section 3) AAO 2011-2012
•Internet