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A presentation on the various theories about light from a historical prespective
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A Biography of LightA Biography of Light
What is Reality?What is Reality?
Corpuscular TheoryCorpuscular Theory In the 17In the 17thth century, Sir century, Sir
Isaac Newton Isaac Newton conducted conducted experiments to experiments to investigate the nature investigate the nature of lightof light
Seven colours Seven colours identifiedidentified
Explained rectilinear Explained rectilinear propagation, reflection propagation, reflection and refractionand refraction
Velocity predicted to Velocity predicted to be greater in denser be greater in denser mediummedium
Wave theory Light imagined to be
a mechanical longitudinal wave propagating through hypothetical ‘ether’
Interference – Newton’s rings and Thomas Young’s double-slit experiment
Diffraction-bending of light around obstacles; illumination of geometrical shadow
Both explained by superposition principle
More successes of wave theory Huygens - Secondary wavelets emanating
from each point of wave front Polarization and rectilinear propagation
required transverse wave Fresnel explained rectilinear propagation Locus of all particles vibrating in same
phase is wavefront Variation of velocity with medium explained
—slower in denser medium
Maxwell’s unification James Clerk Maxwell unifies electricity
and magnetism Predicts an electromagnetic
wave,travelling with velocity of light No apparent medium for propagation
Propagation of light Electromagnetic wave is
transverse When either E or B
varies with time,the other is induced in space
Electric and magnetic fields perpendicular to each other and direction of propagation
Varying harmonically Speed depends on
medium
Electromagnetic spectrum
Consists of a continuous spectrum, from radio waves to gamma rays
Visible light ,of wavelengths from 350 to 750 nm
Polarization
Vibration of fields confined to a single plane perpendicular to direction of propagation
Linear, elliptical or circular polarization
Doppler effectDoppler effect► Frequency of light can appear to be shifted Frequency of light can appear to be shifted
depending on relative motion of source and depending on relative motion of source and observerobserver
► Blue-shift if motion towards each otherBlue-shift if motion towards each other► Red-shift if away from each otherRed-shift if away from each other
Problems with wave theory
Wave propagated through an all-pervading hypothetical ‘ether’, but undetectable
No reference frame mentioned in Maxwell’s theory
Michelson-Morley experiment failed to detect motion of earth through ether
Michelson-Morley experiment
Attempted to detect motion of earth through ether
Based on an interferometer, to detect path difference between light beams travelling along and across earth’s direction of motion
Path difference expected based on classical theory, but none detected
Relativity
Einstein assumes that speed of light is same for all observers, regardless of their motion
No preferred frame in mechanics, but has to be modified to agree with Maxwell’s electrodynamics
Blackbody radiation Intensity proportional to
fourth-power of temperature
Spread over many wavelengths
Wavelength of max intensity inversely proportional to temperature
Unexplainable by wave theory
Explanation requires hypothesis of light quanta
Quantum era begins
Max Planck assumes that energy can be exchanged only in packets
‘Ultraviolet catastrophe’ averted Einstein uses photons to explain
photoelectric effect Quantization a part of Bohr’s atomic
model
Various types of spectraVarious types of spectra Emission spectrum-Emission spectrum-
light emitted by light emitted by materials is of materials is of characteristic characteristic wavelength.wavelength.
Photons are emitted Photons are emitted when electron makes when electron makes transition from higher transition from higher to lower levelto lower level
Absorption spectra-Absorption spectra-certain wavelengths in certain wavelengths in incident light are incident light are absorbed and are absorbed and are missing in spectrummissing in spectrum
Photons absorbed Photons absorbed when electrons jump when electrons jump from lower to higher from lower to higher levellevel
Raman effect Scattered light
from a molecule consists of wavelengths other than the incident one
Raman shift is characteristic of each molecule
Explained fully only by quantum theory
Electrons emitted when light of frequency above critical frequency strikes certain metal surfaces
Number of electrons ejected is proportional to intensity but independent of frequency.
No time interval between arrival of light and emission of photoelectrons.
Cannot be explained by wave theory.
Theoretically explained by Einstein.
Photoelectric effect
Compton Compton effect- when X effect- when X rays interact rays interact with with electrons, electrons, wave length wave length changes.changes.
Not explained Not explained using wave using wave theory. theory.
More support for particle theory.
Complementary
Certain phenomena require wave picture while others require particle picture.
Wave-particle duality not only for light,but also all matter---de Broglie wavelength
Light quanta are called photons, no rest mass.
Investigation of nature of light has led to advances in many areas of physics
Light is the unifying thread running through all branches of physics
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