A STUDY OF LIGHTA STUDY OF LIGHT

TOPICSTOPICS

HistoryHistoryWhat is a scientific modelWhat is a scientific modelWhy use a model to study light Why use a model to study light Bases of models of lightBases of models of lightWhat models are usedWhat models are usedParticle modelParticle modelUsing the particle modelUsing the particle modelDirect evidenceDirect evidenceProblems Problems

HistoryHistory

1.1. PYTHAGORASPYTHAGORAS (572BC – 490 BC)(572BC – 490 BC)

Believed that light traveled Believed that light traveled as a stream of fast-moving as a stream of fast-moving particles.particles.

2.2. EMPEDOCLES EMPEDOCLES (490 BC – 430 BC)(490 BC – 430 BC)Believed light traveled as Believed light traveled as a wave-like disturbance.a wave-like disturbance.

3.3. PLATOPLATO(424 BC – 346 BC)(424 BC – 346 BC)Light emitted by “streamers” Light emitted by “streamers”

from the eyes.from the eyes.4. 4. EUCLIDEUCLID(323 BC–283 BC)(323 BC–283 BC)Agreed with PlatoAgreed with Plato““How else can we explain How else can we explain

that we do not see a that we do not see a needle on the floor until needle on the floor until our eyes fall upon it?”our eyes fall upon it?”

5. 5. ISAAC NEWTONISAAC NEWTON

(1643 – 1727)(1643 – 1727)

Corpuscular model of lightCorpuscular model of light

6. 6. CHRISTIAAN HUYGENSCHRISTIAAN HUYGENS

(1629 – 1695)(1629 – 1695)

Wave model of lightWave model of light

What is a scientific model?What is a scientific model?

An educated guess about the structure of An educated guess about the structure of something. something.

- Based on known properties- Based on known properties- used to explain known properties in - used to explain known properties in

simple simple termsterms-used to predict behaviors-used to predict behaviors- believed to be true and - believed to be true and

added/adjusted as new information added/adjusted as new information becomes availablebecomes available

Why use a model to study lightWhy use a model to study light

Difficult to study since it Difficult to study since it travels at such a high travels at such a high speed.speed.

Can only be noticed if it Can only be noticed if it reflects off of reflects off of something in its way.something in its way.

An example of a scientific model:An example of a scientific model:

E.g.. Bohr-Rutherford E.g.. Bohr-Rutherford model of an atom model of an atom

vs. vs.

today’s understanding today’s understanding of an atom.of an atom.

Properties of lightProperties of light

Rectilinear PropagationRectilinear Propagation

Reflection – regularReflection – regular - diffuse- diffuse

RefractionRefraction

Partial ReflectionPartial Reflection

Snell’s LawSnell’s Law

nn11sinsinθθ1 1 = n= n22sinsinθθ22

Increased distance Increased distance αα decreased Intensitydecreased Intensity

Light has energyLight has energy

DispersionDispersion

White light can be White light can be separated into it’s separated into it’s component colors by component colors by a prisma prism

Models of LightModels of Light

1.1. Particle Particle

2.2. WaveWave

3.3. Wave-Particle DualityWave-Particle Duality

4.4. Electro-Magnetic WaveElectro-Magnetic Wave

Particle (Corpuscular) ModelParticle (Corpuscular) Model

Newton proposed that light consisted of Newton proposed that light consisted of streams of tiny particle (corpuscles) streams of tiny particle (corpuscles) emitted like bullets from the light source)emitted like bullets from the light source)

1. Light of the same color is made up of 1. Light of the same color is made up of identical particlesidentical particles

2. Light travels in straight lines (rectilinear 2. Light travels in straight lines (rectilinear propagation)propagation)

3. Particles of light are massless3. Particles of light are massless4. Particles travel at very high speeds4. Particles travel at very high speeds5. Particles of light carry kinetic energy 5. Particles of light carry kinetic energy 6. Light spreads out in uniform distribution 6. Light spreads out in uniform distribution

from the sourcefrom the source7.Particles of light experience perfectly 7.Particles of light experience perfectly

elastic collisionselastic collisions

7. Intensity of light = #particles/second7. Intensity of light = #particles/second

8. Light follows the inverse square law.8. Light follows the inverse square law.9. Speed of light is determined by – medium9. Speed of light is determined by – medium - color- color10. All colors travel at same speed in 10. All colors travel at same speed in

vacuum.vacuum.11. Different colors carry different amounts 11. Different colors carry different amounts

of energy.of energy.12. Particles travel at increased speed in 12. Particles travel at increased speed in

denser medium.denser medium.

Using Particles to Explain Using Particles to Explain Properties of LightProperties of Light

Snell’s LawSnell’s Law

Light travels in straight linesLight travels in straight linesDo Particles?Do Particles?

(rectilinear propagation) (rectilinear propagation)

Consider: Throwing a Consider: Throwing a ball to the ground ball to the ground straight down and it straight down and it bounces back straight bounces back straight up.up.

Therefore: “particles” Therefore: “particles” also travel in straight also travel in straight lines just like lightlines just like light

Particles of light are masslessParticles of light are massless

Consider: any black object. It will absorb Consider: any black object. It will absorb light “particles” without increasing masslight “particles” without increasing mass

Therefore: light particles are virtually Therefore: light particles are virtually masslessmassless

Intensity of light = # of light particles/secondIntensity of light = # of light particles/second

if we expose a piece if we expose a piece of of photographic photographic paperpaper to a short burst to a short burst of light we will see:of light we will see:

Is this true for particles?Is this true for particles?

Consider: a spray can of paintConsider: a spray can of paint

If we spray just a If we spray just a short short

burstburst, we get just a , we get just a few few

spotsspots on the screen: on the screen:

The longer we spray, The longer we spray,

the more spots appear the more spots appear

until the whole area is until the whole area is

covered in paint:covered in paint:

Light Particles travel at very high Light Particles travel at very high speeds. Do real particles?speeds. Do real particles?

Light follows the inverse square lawLight follows the inverse square law

Do particles follow the Inverse Do particles follow the Inverse Square Law?Square Law?

Consider: Butter GunConsider: Butter Gun

                                                                                                 

            

All colors travel at same speed in All colors travel at same speed in vacuum.vacuum.

Particles exert a force. Does Light Particles exert a force. Does Light exert a force?exert a force?

Consider: a comet’s tail Consider: a comet’s tail always points away always points away from the sun.from the sun.

Can we explain this in Can we explain this in terms of light terms of light particles?particles?

Problems with the Particle Model Problems with the Particle Model

1.1. Partial ReflectionPartial Reflection

Newton said that the particles arrived in Newton said that the particles arrived in “fits” at the boundary between two “fits” at the boundary between two media.media.

2. Snell’s Law2. Snell’s Law

Particle Model predicts that the speed of Particle Model predicts that the speed of light in water is 4.0 x 10 light in water is 4.0 x 10 88 m/s m/s

Armand Fizeau (1862) measured speed of Armand Fizeau (1862) measured speed of light in water = 2.23 x 10 light in water = 2.23 x 10 8 8 m/sm/s

3. Light can be polarized3. Light can be polarized

If light is a particle then If light is a particle then it should be able to it should be able to pass through pass through polarized platespolarized plates