# Reflection of Light Electrons in a material respond to light striking the material Electrons in a material respond to light striking the material The energy.

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Reflection of Light Electrons in a material respond to light striking the material Electrons in a material respond to light striking the material The energy is either absorbed or reemitted The energy is either absorbed or reemitted For shiny surfaces like mirrors the reflection obeys a law For shiny surfaces like mirrors the reflection obeys a law The angle of reflection equals the angle of incidence The angle of reflection equals the angle of incidence

Law of Reflection

Creating Images

We trace rays from the object We trace rays from the object The image formed by the plane mirror is called a virtual image The image formed by the plane mirror is called a virtual image The image is the same distance behind the mirror as the object is in front of it The image is the same distance behind the mirror as the object is in front of it The object and the image are the same size The object and the image are the same size

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

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

Diffuse Reflection Bumps on the surface must be less than about 1/8 the wavelength of light in order to have a smooth surface Bumps on the surface must be less than about 1/8 the wavelength of light in order to have a smooth surface Big satellite dishes are made of wire mesh and appear smooth to the longer wavelength microwave signals Big satellite dishes are made of wire mesh and appear smooth to the longer wavelength microwave signals

Satellite Dish

Refraction

Refraction Recall that when light passes from one material to another, it is bent because the speed of light is different in the two materials Recall that when light passes from one material to another, it is bent because the speed of light is different in the two materials Put a stick into water as an example Put a stick into water as an example The difference in speed between a material and a vacuum is called index of refraction The difference in speed between a material and a vacuum is called index of refraction

Index of Refraction

Refraction When light slows down at a boundary, it bends toward the normal When light slows down at a boundary, it bends toward the normal When light speeds up at a boundary, it bends away from the normal When light speeds up at a boundary, it bends away from the normal The sines of the angles are related to the two indices of refraction The sines of the angles are related to the two indices of refraction

Refraction This bending of light can play tricks on our minds This bending of light can play tricks on our minds

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

Dispersion The resonant frequencies where light matches electronic motions tend to be at the high end of the spectrum (violet) The resonant frequencies where light matches electronic motions tend to be at the high end of the spectrum (violet) The result is that higher frequencies get trapped in electronic motion and then reemitted after a delay The result is that higher frequencies get trapped in electronic motion and then reemitted after a delay High frequencies travel slower than low frequencies High frequencies travel slower than low frequencies

Dispersion

Dispersion through a Prism

Dispersion though a Prism

Rainbows

Rainbows

Rainbows

Rainbows

Total Internal Reflection At an interface, when light is going from a region of high refractive index (lower speed) to lower index, the light is bent away from the normal At an interface, when light is going from a region of high refractive index (lower speed) to lower index, the light is bent away from the normal If the angle of incidence gets great enough it will be bending away at 90 o If the angle of incidence gets great enough it will be bending away at 90 o This is called the critical angle This is called the critical angle

Total Internal Reflection Once the angle of incidence is larger than the critical angle, the light cannot escape the higher index material Once the angle of incidence is larger than the critical angle, the light cannot escape the higher index material This means that all the light is reflected from the surface back into the higher index material This means that all the light is reflected from the surface back into the higher index material This is total internal reflection This is total internal reflection

Total 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 Reflection By using high index of refraction glass, we can make prisms that exhibit total internal reflection By using high index of refraction glass, we can make prisms that exhibit total internal reflection We can do all sorts of interesting direction changes of light by using these prisms We can do all sorts of interesting direction changes of light by using these prisms

Prisms

Prisms

Prisms When 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.

Prisms

Why Love Diamonds?

Total Internal Reflection in a Diamond

Next time We complete the Chapter We complete the Chapter

Lenses We use the refractive capability of the air- glass interface to bend light We use the refractive capability of the air- glass interface to bend light Lenses control the amount of bending to achieve different results Lenses control the amount of bending to achieve different results Lenses can be either converging or diverging if they are either concave or convex Lenses can be either converging or diverging if they are either concave or convex

Lenses

Lenses

Lens Definitions

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

Image 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 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 Glass Note that flower is inside the focal point of the lens

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

Ray Tracing f f

Lens Defects Distortions caused by lenses are called aberrations Distortions caused by lenses are called aberrations There are several kinds of aberrations There are several kinds of aberrations Correct aberrations by using multiple lenses, often of different kinds of glass Correct aberrations by using multiple lenses, often of different kinds of glass The two most common kinds are spherical and chromatic The two most common kinds are spherical and chromatic

Spherical Aberration

Chromatic Aberration

Wave-Particle Duality This is a shocking topic and a difficult discussion because it shocks your common sense This is a shocking topic and a difficult discussion because it shocks your common sense Recall that Young demonstrated the wave nature of light with the two-slit interference pattern that relied on refraction of light at the edges of the slit Recall that Young demonstrated the wave nature of light with the two-slit interference pattern that relied on refraction of light at the edges of the slit

Wave Particle Duality In 1862, Maxwell published his famous equations describing electric and magnetic fields and showed that EM waves must exist as field induced electric and magnetic fields In 1862, Maxwell published his famous equations describing electric and magnetic fields and showed that EM waves must exist as field induced electric and magnetic fields Thus, he demonstrated mathematically how the EM waves worked. Thus, he demonstrated mathematically how the EM waves worked. In 1887, Heinrich Hertz created radio waves with Maxwell’s wave properties In 1887, Heinrich Hertz created radio waves with Maxwell’s wave properties

Wave Particle Duality So, what is the problem? So, what is the problem? A phenomenon called the photoelectric effect had puzzled physicists for a long time A phenomenon called the photoelectric effect had puzzled physicists for a long time Basically, for some metals, if you shine light on the metal, it will emit electrons Basically, for some metals, if you shine light on the metal, it will emit electrons These electrons are called photoelectrons These electrons are called photoelectrons

Problem is as follows Problem is as follows Light is an EM wave Light is an EM wave The changing electric field shakes electrons in a metal The changing electric field shakes electrons in a metal If you shine the light for a long time,or have a bright enough light, an electron should eventually get enough energy to break free and become a photoelectron If you shine the light for a long time,or have a bright enough light, an electron should eventually get enough energy to break free and become a photoelectron NOT WHAT IS OBSERVED!!! NOT WHAT IS OBSERVED!!! Photoelectric Effect

If the light is of high enough frequency, then photoelectrons appear immediately If the light is of high enough frequency, then photoelectrons appear immediately The number of electrons that appear depends on the intensity of the light The number of electrons that appear depends on the intensity of the light So we have a frequency dependence that we can’t explain by shaking electrons with the electric field So we have a frequency dependence that we can’t explain by shaking electrons with the electric field Photoelectric Effect

What to do? What to do? Einstein said consider light as a stream of particles Einstein said consider light as a stream of particles This is something that Newton had proposed long before, but Young’s experiment made people discard the idea This is something that Newton had proposed long before, but Young’s experiment made people discard the idea Einstein’s light particles are called photons Einstein’s light particles are called photons

Photoelectric Effect So how do we relate properties of the photon to the wavelength or frequency of the light wave So how do we relate properties of the photon to the wavelength or frequency of the light wave Einstein proposed the energy carried by the photon is proportional to the frequency of the light Einstein proposed the energy carried by the photon is proportional to the frequency of the light

Photoelectric Effect This effect requires that photons be completely absorbed and all the energy of the photon be given to the electron This effect requires that photons be completely absorbed and all the energy of the photon be given to the electron Then the electron would have enough energy to escape the metal Then the electron would have enough energy to escape the metal Red light photons had to little energy for the electrons to escape Red light photons had to little energy for the electrons to escape

Photoelectric Effect Blue or violet light photons have higher energy since their frequencies are higher Blue or violet light photons have higher energy since their frequencies are higher Thus producing photoelectrons requires a frequency high enough so that the photons have enough energy to allow the photoelectrons to escape the metal Thus producing photoelectrons requires a frequency high enough so that the photons have enough energy to allow the photoelectrons to escape the metal Dim light would mean fewer photons and thus fewer photoelectrons Dim light would mean fewer photons and thus fewer photoelectrons

Now we have a dilemma Now we have a dilemma The double slit experiment shows that light is a wave The double slit experiment shows that light is a wave The Photoelectric Effect shows that light is a particle The Photoelectric Effect shows that light is a particle Which is it???? Which is it???? Answer: BOTH!!! Answer: BOTH!!! Photoelectric Effect

In order to explain these two experiments we need to invoke the duality of light In order to explain these two experiments we need to invoke the duality of light Light is both a wave and a particle at the same time!!! Light is both a wave and a particle at the same time!!! Defies our common sense, but we must trust the results of experiments, so it must be true Defies our common sense, but we must trust the results of experiments, so it must be true Later we will see that all material particles have wave properties Later we will see that all material particles have wave properties Wave-Particle Duality

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