A Biography of Light

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