Reflection of LightReflection of Light

Electrons in a material respond Electrons in a material respond to light striking the materialto light striking the material

The energy is either absorbed or The energy is either absorbed or reemittedreemitted

For shiny surfaces like mirrors For shiny surfaces like mirrors the reflection obeys a lawthe reflection obeys a law

The angle of reflection equals the The angle of reflection equals the angle of incidenceangle of incidence

Law of ReflectionLaw of Reflection

Creating ImagesCreating Images

Creating ImagesCreating Images

We trace rays from the objectWe trace rays from the object The image formed by the plane The image formed by the plane

mirror is called a mirror is called a virtualvirtual image image The image is the same distance The image is the same distance

behind the mirror as the object is behind the mirror as the object is in front of itin front of it

The object and the image are the The object and the image are the same sizesame size

Curved MirrorsCurved Mirrors

The mirror on the right is used The mirror on the right is used as a makeup or shaving mirror as a makeup or shaving mirror since it magnifiessince it magnifies

Diffuse ReflectionDiffuse Reflection

When the surface is rough, When the surface is rough, images are not formed, but the images are not formed, but the light still reflectslight still reflects

Diffuse ReflectionDiffuse Reflection

Bumps on the surface must be Bumps on the surface must be less than about 1/8 the less than about 1/8 the wavelength of light in order to wavelength of light in order to have a smooth surfacehave a smooth surface

Big satellite dishes are made of Big satellite dishes are made of wire mesh and appear smooth to wire mesh and appear smooth to the longer wavelength microwave the longer wavelength microwave signalssignals

Satellite DishSatellite Dish

Satellite DishSatellite Dish

RefractionRefraction

RefractionRefraction

Recall that when light passes Recall that when light passes from one material to another, it from one material to another, it is bent because the speed of light is bent because the speed of light is different in the two materialsis different in the two materials

Put a stick into water as an Put a stick into water as an exampleexample

The difference in speed between The difference in speed between a material and a vacuum is called a material and a vacuum is called index of refractionindex of refraction

Index of RefractionIndex of Refraction

Index of RefractionIndex of Refraction

Index of RefractionIndex of Refraction

RefractionRefraction

When light slows down at a When light slows down at a boundary, it bends toward the boundary, it bends toward the normalnormal

When light speeds up at a When light speeds up at a boundary, it bends away from the boundary, it bends away from the normalnormal

The sines of the angles are The sines of the angles are related to the two indices of related to the two indices of refractionrefraction

RefractionRefraction

This bending of light can This bending of light can play play trickstricks on our minds on our minds

Another TrickAnother Trick

Light travels faster at higher altitudes because the air is less dense.

DispersionDispersion

The resonant frequencies where The resonant frequencies where light matches electronic motions light matches electronic motions tend to be at the high end of the tend to be at the high end of the spectrum (violet)spectrum (violet)

The result is that higher The result is that higher frequencies get trapped in frequencies get trapped in electronic motion and then electronic motion and then reemitted after a delayreemitted after a delay

High frequencies travel slower High frequencies travel slower than low frequenciesthan low frequencies

DispersionDispersion

Dispersion through a Dispersion through a PrismPrism

Dispersion though a Dispersion though a PrismPrism

RainbowsRainbows

RainbowsRainbows

RainbowsRainbows

RainbowsRainbows

Total Internal ReflectionTotal Internal Reflection

At an interface, when light is At an interface, when light is going from a region of high going from a region of high refractive index (lower speed) to refractive index (lower speed) to lower index, the light is bent lower index, the light is bent away from the normalaway from the normal

If the angle of incidence gets If the angle of incidence gets great enough it will be bending great enough it will be bending away at 90away at 90oo

This is called the This is called the critical anglecritical angle

Total Internal ReflectionTotal Internal Reflection

Once the angle of incidence is Once the angle of incidence is larger than the critical angle, the larger than the critical angle, the light cannot escape the higher light cannot escape the higher index materialindex material

This means that all the light is This means that all the light is reflected from the surface back reflected from the surface back into the higher index materialinto the higher index material

This is This is total internal reflectiontotal internal reflection

Total Internal ReflectionTotal Internal Reflection

This is used in fiber-optic cables to transmit data signals. The light inside the cable cannot escape so no energy (signal) is lost as it travels.

Total Internal ReflectionTotal Internal Reflection

By using high index of refraction By using high index of refraction glass, we can make prisms that glass, we can make prisms that exhibit total internal reflectionexhibit total internal reflection

We can do all sorts of interesting We can do all sorts of interesting direction changes of light by direction changes of light by using these prismsusing these prisms

PrismsPrisms

PrismsPrisms

PrismsPrismsWhen we study lenses, we will see that we can get greater magnification by increasing the path length between lenses.

Here we make binoculars more compact.

PrismsPrisms

Why Love Diamonds?Why Love Diamonds?

Total Internal Reflection in Total Internal Reflection in a Diamonda Diamond

Next timeNext time

We complete the ChapterWe complete the Chapter

LensesLenses

We use the refractive capability We use the refractive capability of the air-glass interface to bend of the air-glass interface to bend lightlight

Lenses control the amount of Lenses control the amount of bending to achieve different bending to achieve different resultsresults

Lenses can be either converging Lenses can be either converging or diverging if they are either or diverging if they are either concave or convexconcave or convex

LensesLenses

LensesLenses

Lens DefinitionsLens Definitions

Image FormationImage Formation

Light from every part of the vase strikes every part of the wall. The result is that no clear image is seen.

Image FormationImage Formation

The pinhole blocks most of the light so that only light from one place on the vase can hit one place on the wall.

Image FormationImage Formation

The lens insures that light from one place on the vase only strikes one place on the wall. But it allows for much more light than the pinhole.

Magnifying GlassMagnifying Glass

Note that flower is inside the focal point of the lens

Converging LensConverging Lens

When the object is outside the focal When the object is outside the focal point, the image appears on the point, the image appears on the other side of the lens and is invertedother side of the lens and is inverted

Ray TracingRay Tracing

ff

qpf

q

p

f

111

distance image

distanceobject

length focal

Lens DefectsLens Defects

Distortions caused by lenses are Distortions caused by lenses are called called aberrationsaberrations

There are several kinds of There are several kinds of aberrationsaberrations

Correct aberrations by using Correct aberrations by using multiple lenses, often of different multiple lenses, often of different kinds of glasskinds of glass

The two most common kinds are The two most common kinds are spherical and chromaticspherical and chromatic

Spherical AberrationSpherical Aberration

Chromatic AberrationChromatic Aberration

Wave-Particle DualityWave-Particle Duality

This is a shocking topic and a This is a shocking topic and a difficult discussion because it shocks difficult discussion because it shocks your common senseyour common sense

Recall that Young demonstrated the Recall that Young demonstrated the wave nature of light with the two-slit wave nature of light with the two-slit interference pattern that relied on interference pattern that relied on refraction of light at the edges of the refraction of light at the edges of the slitslit

Wave Particle DualityWave Particle Duality

In 1862, Maxwell published his famous In 1862, Maxwell published his famous equations describing electric and equations describing electric and magnetic fields and showed that EM magnetic fields and showed that EM waves must exist as field induced waves must exist as field induced electric and magnetic fieldselectric and magnetic fields

Thus, he demonstrated mathematically Thus, he demonstrated mathematically how the EM waves worked.how the EM waves worked.

In 1887, Heinrich Hertz created radio In 1887, Heinrich Hertz created radio waves with Maxwell’s wave propertieswaves with Maxwell’s wave properties

Wave Particle DualityWave Particle Duality

So, what is the problem?So, what is the problem? A phenomenon called the A phenomenon called the

photoelectric effect had puzzled photoelectric effect had puzzled physicists for a long timephysicists for a long time

Basically, for some metals, if you Basically, for some metals, if you shine light on the metal, it will emit shine light on the metal, it will emit electronselectrons

These electrons are called These electrons are called photoelectronsphotoelectrons

Problem is as followsProblem is as follows Light is an EM waveLight is an EM wave The changing electric field shakes The changing electric field shakes

electrons in a metalelectrons in a metal If you shine the light for a long time,or If you shine the light for a long time,or

have a bright enough light, an electron have a bright enough light, an electron should eventually get enough energy to should eventually get enough energy to break free and become a photoelectronbreak free and become a photoelectron

NOT WHAT IS OBSERVED!!!NOT WHAT IS OBSERVED!!!

Photoelectric EffectPhotoelectric Effect

If the light is of high enough If the light is of high enough frequency, then photoelectrons frequency, then photoelectrons appear immediatelyappear immediately

The number of electrons that appear The number of electrons that appear depends on the intensity of the lightdepends on the intensity of the light

So we have a frequency dependence So we have a frequency dependence that we can’t explain by shaking that we can’t explain by shaking electrons with the electric fieldelectrons with the electric field

Photoelectric EffectPhotoelectric Effect

Photoelectric EffectPhotoelectric Effect

What to do?What to do? Einstein said consider light as a Einstein said consider light as a

stream of particlesstream of particles This is something that Newton had This is something that Newton had

proposed long before, but Young’s proposed long before, but Young’s experiment made people discard the experiment made people discard the ideaidea

Einstein’s light particles are called Einstein’s light particles are called photonsphotons

Photoelectric EffectPhotoelectric Effect

So how do we relate properties of So how do we relate properties of the photon to the wavelength or the photon to the wavelength or frequency of the light wavefrequency of the light wave

Einstein proposed the energy Einstein proposed the energy carried by the photon is proportional carried by the photon is proportional to the frequency of the lightto the frequency of the light

Energy frequency

E hf

Photoelectric EffectPhotoelectric Effect

This effect requires that photons be This effect requires that photons be completely absorbed and all the completely absorbed and all the energy of the photon be given to the energy of the photon be given to the electronelectron

Then the electron would have Then the electron would have enough energy to escape the metalenough energy to escape the metal

Red light photons had to little Red light photons had to little energy for the electrons to escapeenergy for the electrons to escape

Photoelectric EffectPhotoelectric Effect

Blue or violet light photons have Blue or violet light photons have higher energy since their higher energy since their frequencies are higherfrequencies are higher

Thus producing photoelectrons Thus producing photoelectrons requires a frequency high enough so requires a frequency high enough so that the photons have enough that the photons have enough energy to allow the photoelectrons energy to allow the photoelectrons to escape the metalto escape the metal

Dim light would mean fewer photons Dim light would mean fewer photons and thus fewer photoelectronsand thus fewer photoelectrons

Now we have a dilemmaNow we have a dilemma The double slit experiment shows The double slit experiment shows

that light is a wavethat light is a wave The Photoelectric Effect shows that The Photoelectric Effect shows that

light is a particlelight is a particle Which is it????Which is it???? Answer: BOTH!!!Answer: BOTH!!!

Photoelectric EffectPhotoelectric Effect

In order to explain these two In order to explain these two experiments we need to invoke the experiments we need to invoke the dualityduality of light of light

Light is both a wave and a particle Light is both a wave and a particle at the same time!!!at the same time!!!

Defies our common sense, but we Defies our common sense, but we must trust the results of must trust the results of experiments, so it must be trueexperiments, so it must be true

Later we will see that all material Later we will see that all material particles have wave propertiesparticles have wave properties

Wave-Particle DualityWave-Particle Duality