scattering and absorption of light

SCATTERING & ABSORPTION OF LIGHT

BIKASH SAPKOTABACHELOR OF OPTOMETRY16TH BATCHMAHARAJGUNJ MEDICAL CAMPUS,NEPAL

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 blueo 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 sunseto 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 fibero Involves scattering of light: due to change in

local refractive indexo 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 edemao 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 black body

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 radiationo Lens: Near UV (315-390 nm) IR > 2500 nm UV absorption by lens increases with the increasing ageo 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 selectiveo Some lenses absorb mostly in the IR region of spectrum.

E.g. Calobar, Ray Bano Other absorb in UV region. E.g. Spectacle Pink, UV 400,

UV 530o 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

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