THE 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 light Propagation-within a uniform medium is along a straight line Reflection-occurs at the surface/boundary of a medium Refraction(or bending)-may occur where a change of speed is experienced Interference-found where two waves are superposed Diffraction(or bending around corners)-takes place when waves pass the edges of obstructions

THE WAVE & PARTICULATE THEORY OF LIGHT

Light: Particle or Wave? Light is a form of energy. Energy can be transferred from one 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) Theory Huygens’(hi-ganz) Wave Theory Maxwell’s Electromagnetic Theory Planck’s Quantum Theory

NEWTON’S CORPUSCULAR THEORY Picture of Sir Isaac Newton

NEWTON’S CORPUSCULAR THEORY According to Sir Isaac Newton’s Corpuscular Theory, a luminous body continuously emitting tiny light and elastic particles called corpuscles in all directions. When these particles fall on the retina of the eye, they produce the sensation of vision. This theory can explain a number of phenomena concerning light like reflection and refraction. Reflection was explained by assuming that the corpuscles which fall on a smooth surface would bounce back like rubber balls hitting on a wall. When this theory was used to explain refraction scientists found that the velocity of light in a denser medium would be more than that in a rarer medium.

Picture Steel 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’ WAVE THEORY Picture of Christiaan Huygens. He was a Dutch mathematician, physicist and astronomer. DOB: ~95

Huygens’ Wave Theory Christiaan Huygens proposed the wave theory of light. According to this a luminous body is a source of disturbance in a hypothetical medium called ether. This disturbance is propagated in the forms of waves through space and the energy is distributes equally 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.

Picture The 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’S ELECTROMAGNETIC THEORY Picture of James Clerk Maxwell He was a Scottish mathematical physicist. D.O.B:

Maxwell’s Electromagnetic Theory Electromagnetic Theory of light was put forward by James Clerk Maxwell in According to this theory light consists of fluctuating electric and magnetic propagating in the form of electromagnetic waves. But this theory fail to explain the photoelectric effect(occurs when 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.

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PLANCK’S QUANTUM THEORY Picture of Max Planck He was a German theoretical physicist. D.O.B.:

Planck’s Quantum Theory According to Max Planck's quantum theory, Radiation is not continuous but is made up of tiny packets of energy called photons. However, this theory Fail 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 theory Arguments favoring both a particulate and wave theory were plausible when applied to the properties of light observed in the 17 th 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 Questions What 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?

References boomeria.org/physicstextbook/ch12.html hibp.ecse.pl.edu chandra.harvard.edu/resources/glossaryE.html astronomy.swin.edu.au tutorvista.com/content/physics/physics-ii/light- reflection/light-nature-theories.php

Group members Rashana Anderson Natasha Bando Colleen Lyngo Zhanee Marley Toni-Ann Miller