LIGHTTHE THEORY OF THE NATURE OF LIGHT

What is light?Light is an electromagnetic

radiation(radiation consisting of waves of energy associated with electric and magnetic fields resulting from the acceleration of an electric charge).

It is a stream of photons(packets of energy released when the electrons fall).

Properties of lightPropagation-within a uniform medium is

along a straight lineReflection-occurs at the surface/boundary of

a mediumRefraction(or bending)-may occur where a

change of speed is experiencedInterference-found where two waves are

superposedDiffraction(or bending around corners)-

takes place when waves pass the edges of obstructions

THE NATURE OF LIGHTTHE WAVE & PARTICULATE THEORY OF

LIGHT

Light: Particle or Wave?Light is a form of energy. Energy can be transferred

fromone point to another point either by particle motion or by wave motion. Accordingly, different theories on the nature of light have been proposed. The important theories are as follows:-Newton’s Corpuscular (Particle) TheoryHuygens’(hi-ganz) Wave TheoryMaxwell’s Electromagnetic TheoryPlanck’s Quantum Theory

NEWTON’SCORPUSCULAR THEORY

Picture of Sir Isaac Newton

NEWTON’S CORPUSCULAR THEORY According to Sir Isaac Newton’s Corpuscular Theory, a luminous body continuously emitting tiny lightand elastic particles called corpuscles in all directions.When these particles fall on the retina of the eye, theyproduce the sensation of vision.

This theory can explain a number of phenomenaconcerning light like reflection and refraction. Reflectionwas explained by assuming that the corpuscles whichfall on a smooth surface would bounce back like rubberballs hitting on a wall. When this theory was used to explainrefraction scientists found that the velocity of light in a

densermedium would be more than that in a rarer medium.

PictureSteel ball bearings thrown against a smooth steel plate rebound in much the same way light is reflected. Perfectly elastic particles rebounding from a resilient surface, then, could provide a suitable model for the reflection of light. See Figure 12-2

HUYGENS’WAVETHEORY

Picture of Christiaan Huygens.

He was a Dutch mathematician, physicist and astronomer.

DOB: 1629-1~95

Huygens’ Wave TheoryChristiaan Huygens proposed the wave

theory of light. According to this a luminous body is asource of disturbance in a hypothetical medium called ether. This disturbance is propagated in the forms of waves through space and the energy is distributesequally in all directions.

His concept, published in 1690 and now recognized as Huygens' principle, may be stated as follows: Each point on a wave front may be regarded as a new source of disturbance.

PictureThe wave theory treats light as a train of waves having wave fronts perpendicular to the paths of the light rays. The light energy is considered to be distributed uniformly over the advancing wave front. Huygens thought of a ray merely as a line of direction of waves propagated from a light source.

MAXWELL’SELECTROMAGNETICTHEORY

Picture of James Clerk Maxwell

He was a Scottish mathematical physicist.

D.O.B: 1831-1879

Maxwell’s Electromagnetic TheoryElectromagnetic Theory of light was put forward

by James Clerk Maxwell in 1873. According to thistheory light consists of fluctuating electric and magneticpropagating in the form of electromagnetic waves. But this theory fail to explain the photoelectric effect(occurswhen photons of light produce free electrons).Maxwell determined that the energy of an

electromagnetic wave is equally divided between an electric field and a magnetic field, each perpendicular to the other, and both perpendicular to the direction of propagation of the wave.

Pictures

PLANCK’SQUANTUMTHEORY

Picture of Max Planck

He was a German theoretical physicist.

D.O.B.: 1858-1947

Planck’s Quantum TheoryAccording to Max Planck's quantum

theory, Radiation is not continuous but is made up of

tiny packets of energy called photons. However, this

theoryFail to explain other optical phenomena.He postulated that light is radiated and

absorbed in indivisible packets, or quanta. We now call these packets, or quanta, photons.

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Summary of the debate of the wave and particle theoryArguments favoring both a particulate and wave

theory were plausible when applied to the properties of light observed in the 17th century. Because of the plausibility of both theories a scientific debate concerning the nature of light developed and continued for more than a century.

From all the above theories it is clear that certain optical phenomena can be explained clearly only if light is considered to be made up of particles, while certain other phenomena can be explained only if we consider light as a wave. Thus light appears to have a dual nature.

Review QuestionsWhat is light?What was argued among physicists for many

years?Name the four (4) theories of light.Explain each theory.Can it be concluded then, that light is either

a particle, wave or both a wave and particle?

Referenceswww.darwinistsdilemma.comwww.ufoprovo.blogspot.comboomeria.org/physicstextbook/ch12.htmlhibp.ecse.pl.educhandra.harvard.edu/resources/

glossaryE.htmlastronomy.swin.edu.aututorvista.com/content/physics/physics-ii/

light-reflection/light-nature-theories.php

Group membersRashana Anderson

Natasha Bando

Colleen Lyngo

Zhanee Marley

Toni-Ann Miller