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# Unit 2 Notes

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• That is, as the angle of incidence is increased, the brightness of the refracted ray decreases and the brightness of the reflected ray increases . Finally, we would observe that the angles of the reflection and refraction are not equal. Since the light waves would refract away from the normal (a case of the SFA principle of refraction), the angle of refraction would be greater than the angle of incidence. And if this is the case, the angle of refraction would also be greater than the angle of reflection (since the angles of reflection and incidence are the same). As the angle of incidence is increased, the angle of refraction would eventually reach a 90-degree angle. These principles are depicted in the diagram below.
• The mechanism of energy transport through a medium involves the absorbtion and re-emission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorbtion of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is re-emitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then re-emitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorbtion and re-emission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorbtion and re-emission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.
• The mechanism of energy transport through a medium involves the absorbtion and re-emission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorbtion of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is re-emitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then re-emitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorbtion and re-emission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorbtion and re-emission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.
• The mechanism of energy transport through a medium involves the absorbtion and re-emission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorbtion of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is re-emitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then re-emitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorbtion and re-emission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorbtion and re-emission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.
• The mechanism of energy transport through a medium involves the absorbtion and re-emission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorbtion of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is re-emitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then re-emitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorbtion and re-emission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorbtion and re-emission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.
• The mechanism of energy transport through a medium involves the absorbtion and re-emission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorbtion of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is re-emitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then re-emitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorbtion and re-emission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorbtion and re-emission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.
• The mechanism of energy transport through a medium involves the absorbtion and re-emission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorbtion of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is re-emitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then re-emitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorbtion and re-emission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorbtion and re-emission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.
• The mechanism of energy transport through a medium involves the absorbtion and re-emission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorbtion of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is re-emitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then re-emitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorbtion and re-emission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorbtion and re-emission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.

## Unit 2 NotesPresentation Transcript

• Physics 30SUnit 2The Nature of Light
• Unit 2 – The Nature of Light
• Unit 2 – The Nature of LightInference is the act or process of deriving aconclusion based solely on what one alreadyknows.A model is a pattern, plan, representation(especially in miniature), or description designedto show the main object or workings of an object,system, or concept.In science a theory is a testable model of themanner of interaction of a set of naturalphenomena, capable of predicting futureoccurrences or observations of the same kind,and capable of being tested through experimentor otherwise verified through empiricalobservation.
• Unit 2 – The Nature of LightModels, Theories & Laws• Scientists give the title law to certain concise but general statements about how nature behaves.• The law is a summary of the results of our observations. It is a description of nature without an explanation of why nature behaves that way.• To be called a law, a statement must be found experimentally valid over a wide range of observed phenomena.• The law in a sense bring unity to many observations.• For less general statements, the term principle is often used.
• Unit 2 – The Nature of LightModels, Theories & Laws• Scientific observations and laws are like pieces of a jigsaw puzzle. When enough pieces have fallen into place, a meaningful pattern emerges. This pattern is a theory.• A theory provides a general explanation for the observations, and is created to explain these observations. These observations may be made by many scientists over a long period of time.• Theories are inspirations that come from the minds of human beings.• An important aspect of any theory is how well it can quantitatively predict phenomena.
• Unit 2 – The Nature of LightModels, Theories & Laws• Scientists often make use of models to help them interpret their observations. A scientific model is not like a model airplane or a globe.• A scientific model is a mental picture that helps us understand something we cannot see or experience directly. The model may include an analogy with something we are already familiar with.• The purpose of a model is to give us a mental or visual picture, something to hold onto, when we cannot see what is actually happening.• Models give us a deeper understanding.• Scientific models change as new discoveries are made.
• Unit 2 – The Nature of LightModeling Video
• Unit 2 – The Nature of LightBlack Box Activity
• Unit 2 – The Nature of LightThe Scientific Method• The scientific method is an orderly and systematic approach to gather knowledge. With this approach, new ideas about the world are constantly checked against reality. We can also think of the scientific method as a way of answering questions about the world we live in.
• Unit 2 – The Nature of LightThe Scientific Method
• Unit 2 – The Nature of Light• A brief outline of the scientific method might include the following. – First the scientist makes observations, or records facts of what is seen. – The observation leads to a question. (Sometimes the question may come before the observations are made.) – Thinking about the question produces a hypothesis, a tentative answer to the question. – The scientist tests the hypothesis with a carefully designed procedure, an experiment. As part of the experiment, the scientist carefully records and analyzes data, or information, gathered in the experiment. – The experiment produces a result, or conclusion, which the scientists interprets carefully. From the interpretation, or inference, the result may raise new questions and lead to new hypotheses and new experiments.
• Unit 2 – The Nature of Light– After a number of experiments, the scientist may be able to summarize the results in a natural law, which describes how nature behaves but does not explain why nature behaves in that particular way. It often takes a mathematical form.– Finally the scientist may be able to formulate a theory. The theory explains why nature behaves in the way described by the natural law. It answers the original questions and also any other questions that were raised during the process. The theory also predicts the results of further experiments and this is how the theory is checked. A model could also be formed at this point along with or instead of the theory. The model is a mental picture that helps us to understand what it is that we are observing
• Unit 2 – The Nature of LightThe Scientific Method "Im All Thumbs"• What makes a "Class Champion" thumb wrestler? Does thumb diameter, length, or wrist diameter have an effect on the overall chances of winning a thumb wrestling match? In this investigation we will develop a hypothesis based on physical data collected from our classmates. We will then test this hypothesis by conducting a thumb wrestling tournament to determine an overall "Class Champion".
• Unit 2 – The Nature of LightThe Scientific Method
• Unit 2 – The Nature of LightKnowledge Claims“I believe that Coke is the best soft drink.”• A knowledge claim is a declaration of conviction consisting of a sentence of the type "I know that … " or "I believe that".
• Unit 2 – The Nature of LightKnowledge Claims• Knowledge claims are supported by evidence whose nature depends on the training and the experiment of the claimer. Evidence can be first hand observations, deference to authority, or plausible explanations. Deference to authority can range from naive acceptance of the authority to a more careful consideration of evidence.• To be convincing, the claimer must formulate a relevant argument with the intended audience. Sometimes the evidence is given in the form of a critical experiment that is overwhelmingly convincing. An argument based on good evidence can be referred to as evidential argument.
• Unit 2 – The Nature of LightKnowledge Claims• Knowledge claims are supported by evidence whose nature depends on the training and the experiment of the claimer. Evidence can be first hand observations, deference to authority, or plausible explanations. Deference to authority can range from naive acceptance of the authority to a more careful consideration of evidence.• To be convincing, the claimer must formulate a relevant argument with the intended audience. Sometimes the evidence is given in the form of a critical experiment that is overwhelmingly convincing. An argument based on good evidence can be referred to as evidential argument.
• Unit 2 – The Nature of LightModels of Light
• Unit 2 – The Nature of LightTheories of Light• In the seventeenth century two rival theories of the nature of light were proposed, the wave theory and the corpuscular theory. The Dutch astronomer Huygens (1629-1695) proposed a wave theory of light. He believed that light was a longitudinal wave, and that this wave was propagated through a material called the aether. Since light can pass through a vacuum and travels very fast Huygens had to propose some rater strange properties for the aether: for example; it must fill all space and be weightless and invisible. For this reason scientists were sceptical of his theory.
• Unit 2 – The Nature of LightTheories of Light• In 1690 Newton proposed the corpuscular theory of light. He believed that light was shot out from a source in small particles, and this view was accepted for over a hundred years.• The quantum theory put forward by Max Planck in 1900 combined the wave theory and the particle theory, and showed that light can sometimes behave like a particle and sometimes like a wave. You can find a much fuller consideration of this in the section on the quantum theory.
• Unit 2 – The Nature of LightModels of Light
• Unit 2 – The Nature of LightNewton’s Model of Light• Isaac Newton imagined light as streams of tiny particles he called corpuscles that shoot out like bullets from light sources. Newton’s theory is commonly called the Corpuscular Theory of the Particle Theory of light.
• Unit 2 – The Nature of LightHuygen’s Model of Light• The wave theory was first proposed by Robert Hooke in 1665 and then improved upon by Christian Huygens. The Wave Theory of Light tried to explain the following properties of light.
• Unit 2 – The Nature of LightNewton’s Model of Light• Newton proposed his theory and gave the following arguments to support his theory. – Rectilinear Propagation - Shadows and light rays traveling through the clouds showed that light traveled in straight lines. We know that a bullet shot from a gun will curve due to the force of gravity. Newton argued that light particles did not do this because the traveled at extremely high speeds. He also argued that they must have an extremely small mass because they did not exert any noticeable pressure.
• Unit 2 – The Nature of LightNewton’s Model of Light• Rectilinear Propagation
• Unit 2 – The Nature of LightNewton’s Model of Light• Newton proposed his theory and gave the following arguments to support his theory. – Reflection - Newton demonstrated that in a perfectly elastic collision between a particle and a solid surface, the incident velocity equals the magnitude of the reflected velocity and the angle of incidence equals the angle of reflection.
• Unit 2 – The Nature of LightNewton’s Model of Light• Reflection
• Unit 2 – The Nature of LightNewton’s Model of Light• Newton proposed his theory and gave the following arguments to support his theory. – Refraction - To explain refraction Newton believed that water attracts approaching particles of light much in the same way as gravity attracts a rolling ball on an incline. He predicted that light would speed up as it enters a medium with a higher index of refraction such as water.
• Unit 2 – The Nature of LightNewton’s Model of Light• Refraction
• Unit 2 – The Nature of LightNewton’s Model of Light• Newton proposed his theory and gave the following arguments to support his theory. – Dispersion - The dispersion of light had been observed for year. Newton explained this phenomenon by attributing the different amounts of refraction to the different masses of the particles. He said the violet particles had less mass than the red particles, therefore showing a greater refraction.
• Unit 2 – The Nature of LightNewton’s Model of Light• Dispersion
• Unit 2 – The Nature of LightNewton’s Model of Light• Newton proposed his theory and gave the following arguments to support his theory. – Diffraction - Newton explained the diffraction that occurred when light passed through two slits was the result of the particles colliding with one another and with the edges of the slit.
• Unit 2 – The Nature of LightNewton’s Model of Light• Newton proposed his theory and gave the following arguments to support his theory. – Partial Reflection/Partial Refraction - When light refracts, some of the light is also reflected. Newton had difficulty explaining this part.
• Unit 2 – The Nature of LightHuygen’s Model of Light• The wave theory was first proposed by Robert Hooke in 1665 and then improved upon by Christian Huygens. The Wave Theory of Light tried to explain the following properties of light.
• Unit 2 – The Nature of LightHuygen’s Model of Light• Rectilinear Propagation - Early attempts to explain rectilinear propagation proved difficult. Huygens thought of light rays as the direction of travel of the wave (wave ray).
• Unit 2 – The Nature of LightHuygen’s Model of Light• Diffraction - Waves diffract and so did light. Huygen’s principle is consistent with diffraction of waves. It wasn’t until the size of the wavelengths of light was discovered that Huygens principle was proven .
• Unit 2 – The Nature of LightHuygen’s Model of Light• Reflection - The same argument Newton used holds true for waves. Waves obey the laws of reflection.
• Unit 2 – The Nature of LightHuygen’s Model of Light• Refraction - Huygen was able to predict that light is bent towards the normal as it enters a more optically dense material. This indicates that light slows down as it enters water. This was not proven till years later.
• Unit 2 – The Nature of LightHuygen’s Model of Light• Refraction
• Unit 2 – The Nature of LightHuygen’s Model of Light• Partial Reflection/ Partial Refraction Waves partially reflect and partially refract when there is a change in velocity. The amount of partial reflection varies with the angle of incidence. There is a critical angle for waves and light. Angles of incidence greater than the critical angle produce only reflection.
• Unit 2 – The Nature of Light
• Unit 2 – The Nature of LightLight & Refraction• A wave doesnt just stop when it reaches the end of the medium. The transmitted wave undergoes refraction (or bending) if it approaches the boundary at an angle.• Refraction is the bending of the path of a light wave as it passes from one material to another material. The refraction occurs at the boundary and is caused by a change in the speed of the light wave upon crossing the boundary.
• Unit 2 – The Nature of LightLight & Refraction• The tendency of a ray of light to bend one direction or another is dependent upon whether the light wave speeds up or slows down upon crossing the boundary.
• Unit 2 – The Nature of LightLight & Refraction• FST = Fast to Slow, Towards Normal – If a ray of light passes across the boundary from a material in which it travels fast into a material in which travels slower, then the light ray will bend towards the normal line.• SFA = Slow to Fast, Away From Normal – If a ray of light passes across the boundary from a material in which it travels slow into a material in which travels faster, then the light ray will bend away from the normal line.
• Unit 2 – The Nature of LightSnell’ Law & Angle of Refraction• There is a mathematical equation relating the angles which the light rays make with the normal to the indices (plural for index) of refraction of the two materials on each side of the boundary. This mathematical equation is known as Snells Law.
• Unit 2 – The Nature of LightSnell’ Law & Angle of Refraction• Snells law will apply to the refraction of light in any situation, regardless of what the two media are.• The study of the refraction of light as it crosses from one material into a second material yields a general relationship between the sines of the angle of incidence and the angle of refraction. This general relationship is expressed by the following equation: ni *sin(θ i) = nr * sin(θ r)
• Unit 2 – The Nature of LightSnell’ Law & Angle of Refraction ni * sin(θ i) = nr * sin(θ r) – ("θ i") = angle of incidence – ("θ r") = angle of refraction – ni = index of refraction of the incident medium – nr = index of refraction of the refractive medium
• Unit 2 – The Nature of LightSnell’ Law & Angle of Refraction
• Unit 2 – The Nature of LightSnell’ Law & Angle of Refraction• Measure, calculate, and draw in the refracted ray with the calculated angle of refraction.
• Unit 2 – The Nature of LightSnell’ Law & Angle of Refraction• Answer
• Unit 2 – The Nature of LightRefractive Index• In 1621 Snell found that when the sin of the angle of incidence is divided by the sin of the angle of refraction, the result is a constant value for a given interface (boundary layer between specific materials).• This constant number was named the Relative Refractive Index and is specific for each pair of media.• For an air-to-water interface, the relative refractive index is 1.33.
• Unit 2 – The Nature of LightRefractive Index• A more useful quantity is the Absolute Refractive Index. This is the relative refractive index for a vacuum-to- medium interface and is specific for each medium. The value of the index for light traveling from air into the substance is so close to the value of absolute refractive index that we only distinguish between them in rare instances.
• Unit 2 – The Nature of LightSnell’ Law• Lab 17.2• Worksheet 2.2.1• Worksheet 2.2.3• Worksheet 2.2.4
• Unit 2 – The Nature of LightTotal Internal reflection• Total internal reflection is the reflection of the total amount of incident light at the boundary between two medium.
• Unit 2 – The Nature of LightTotal Internal reflection• At angle of incidence close to 0 degrees, most of the light energy is transmitted across the boundary and very little of it is reflected. As the angle is increased to greater and greater angles, we would begin to observe less refraction and more reflection.
• Unit 2 – The Nature of LightTotal Internal reflection
• Unit 2 – The Nature of LightTotal Internal reflection• The maximum possible angle of refraction is 90-degrees. There is some specific value for the angle of incidence (called the "critical angle") which yields an angle of refraction of 90-degrees.• This particular value for the angle of incidence could be calculated using Snells Law (ni = 1.33, nr = 1.000, = 90 degrees, = ???) and would be found to be 48.6 degrees. Any angle of incidence which is greater than 48.6 degrees would not result in any refraction.• Instead, when the angles of incidence is greater than 48.6 degrees (the critical angle), all of the energy (the total energy) carried by the incident wave to the boundary stays within the water (internal to the original medium) and undergoes reflection off the boundary. When this happens, total internal reflection occurs.
• Unit 2 – The Nature of LightTotal Internal reflection• Total internal reflection (TIR) is the phenomenon which involves the reflection of all the incident light off the boundary.
• Unit 2 – The Nature of LightTotal Internal reflection• TIR only takes place when both of the following two conditions are met: – the light is in the more dense medium and approaching the less dense medium. – the angle of incidence is greater than the so- called critical angle (since light only bends away from the normal when passing from a more dense medium into a less dense medium)• Total internal reflection only occurs with large angles of incidence. The critical angle is different for different media.
• Unit 2 – The Nature of LightTotal Internal reflection
• Unit 2 – The Nature of LightTotal Internal reflection• To calculate the critical angle, a modification of Snell’s Law equation can be used: ni * sineθi = nr * sine θr ni * sineθcrit = nr * sine (90 degrees) ni * sineθcrit = nr sineθcrit = nr/ni θcrit = sine-1 (nr/ni)
• Unit 2 – The Nature of LightYoung’s Experiment• The research by Thomas Young (1773 – 1829) into the interference of light was critical in demonstrating that light has wave like properties. His famous experiment has become known as “Young’s Experiment”.
• Unit 2 – The Nature of LightYoung’s Experiment
• Unit 2 – The Nature of LightYoung’s Experiment –Simulation Activity
• Unit 2 – The Nature of LightYoung’s Experiment• Instead of using two sources, he used only one source of light directing it through two pinholes placed very close together.• The light was diffracted through each pinhole so that each acted as a point source of light.• Since the sources were close together, the spacing between the nodal lines was great enough that the pattern could be seen.• Because the light from the two pinholes came from the same source, the two interfering beams of light were always in phase and a single, fixed interference patter could be created on the screen.• A series of bright and dark bands called interference fringes were produced.
• Unit 2 – The Nature of LightYoung’s Experiment• We will look at interference in Young’s Experiment at three different angles. In the diagrams on the following slides, light waves in phase are passing through the slits S1 and S2 which are a distance d apart. The waves spread out in all directions through the slits.
• Unit 2 – The Nature of LightYoung’s Experiment• In this diagram, both waves that reach the centre of the screen are in phase because they travel the same distance. Constructive interference occurs and there is a bright spot on the screen.
• Unit 2 – The Nature of LightYoung’s Experiment• In this diagram, the waves from S2 travel an extra 1/2 λ to reach the screen. When they do so, they are exactly out of phase. This means that the crest from the wave of S1 meets the trough from the wave of S2. Destructive interference occurs and the screen is dark at this point. This corresponds to nodal point 1.
• Unit 2 – The Nature of LightYoung’s Experiment• In this diagram, the point P is moving further away from the center of the screen. A point is reached where the path difference is one wavelength and the waves are once again in phase.
• Unit 2 – The Nature of LightYoungs Experiment and His Useful Equation• For the interference of light from two point sources, the equation below can be used.• In this equation, λ is the wavelength of the light, ∆ x is the distance between adjacent nodal lines on a screen, – d is the separation between the slits – L is the perpendicular distance from the slits to the screen.
• Unit 2 – The Nature of LightElectromagnetic Waves• Electromagnetic waves are waves which can travel through the vacuum of outer space.• Mechanical waves, unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another.• Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.
• Unit 2 – The Nature of LightElectromagnetic Waves• Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 108 m/s (commonly known as "c").• The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 108 m/s.
• Unit 2 – The Nature of LightElectromagnetic Waves• The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 108 m/s.
• Unit 2 – The Nature of LightElectromagnetic Waves• Electromagnetic waves are produced by a vibrating electric charge and as such, they consist of both an electric and a magnetic component.
• Unit 2 – The Nature of LightElectromagnetic Waves
• Unit 2 – The Nature of LightElectromagnetic Spectrum• Electromagnetic waves exist with an enormous range of frequencies. This continuous range of frequencies is known as the electromagnetic spectrum.• The entire range of the spectrum is often broken into specific regions. The subdividing of the entire spectrum into smaller spectra is done mostly on the basis of how each region of electromagnetic waves interacts with matter.
• Unit 2 – The Nature of LightElectromagnetic Spectrum• The longer wavelength, lower frequency regions are located on the far left of the spectrum and the shorter wavelength, higher frequency regions are on the far right. Two very narrow regions with the spectrum are the visible light region and the X-ray region.
• Unit 2 – The Nature of LightVisible Light Spectrum• Though electromagnetic waves exist in a vast range of wavelengths, our eyes are sensitive to only a very narrow band. Since this narrow band of wavelengths is the means by which humans see, we refer to it as the visible light spectrum.• This visible light region consists of a spectrum of wavelengths, which range from approximately 700 nanometers (abbreviated nm) to approximately 400 nm; that would be 7 x 10-7 m to 4 x 10-7 m. This narrow band of visible light is affectionately known as ROYGBIV.
• Unit 2 – The Nature of LightVisible Light Spectrum
• Unit 2 – The Nature of LightThe Photoelectric Effect• Heinrich Hertz noticed that when one exposes certain metal surfaces to an ultraviolet light, negative charges were emitted from the metal. How can we explain the emission of negative charges?What is the Photoelectric Effect?• The photoelectric effect refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light.
• Unit 2 – The Nature of LightThe Photoelectric Effect• Energy contained within the incident light is absorbed by electrons within the metal, giving the electrons sufficient energy to be "knocked" out of, that is, emitted from, the surface of the metal.
• Unit 2 – The Nature of LightThe Photoelectric Effect
• Unit 2 – The Nature of LightThe Photoelectric Effect
• Unit 2 – The Nature of LightThe Photoelectric Effect: WaveModel or Particle Model• The wave model predicts that the energy, which is distributed along the wave, will eventually build up and release a bunch of electrons at the same time. According to the wave model, light of any frequency should demonstrate the photoelectric effect. Lower frequencies would just take longer to build up enough energy to release the electrons. A low frequency wave with high intensity should eventually be able to dislodge an electron. However this is not the case, it is observed that only certain frequencies emit electrons. The wave model fails to accurately predict the photoelectric phenomenon.
• Unit 2 – The Nature of LightThe Photoelectric Effect: WaveModel or Particle Model• Einstein (1905) successfully resolved this paradox by proposing that the incident light consisted of individual quanta, called photons, that interacted with the electrons in the metal like discrete particles, rather than as continuous waves.
• Unit 2 – The Nature of LightThe Photoelectric Effect: WaveModel or Particle Model• Einstein proposed that the light consists of packets of energy called "photons" and that the quantity of energy of each photon is fixed and depends on its frequency.• The higher the frequency, the greater the energy contained in the photon. Thus, the particle model predicts that individual photons knock out electrons and that only photons with enough energy (above the threshold frequency) can do this.• Einstein’s theory explained the existence of a threshold frequency. A photon must have a minimum energy to eject an electron from a metal. This minimum energy depends on the threshold frequency, fo, of the light.
• Unit 2 – The Nature of LightThe Photoelectric Effect: WaveModel or Particle Model• If the photon has a frequency below fo, then it does not have the energy needed to eject an electron. Light with a frequency greater than fo has more energy than needed to eject the electron. The excess energy make the electron move, and is converted to kinetic energy of the moving electron.• When the frequency of the incident light is too low, the photon does not give the absorbing electron sufficient energy and it remains bound to the surface. The intensity (brightness) of the light is only a measure of the rate at which the photons strike the surface, not the energy of the photon. This explains why the kinetic energy of the emitted photoelectrons and the threshold frequency do not depend on the intensity of the incident light.
• Unit 2 – The Nature of LightThe Photoelectric Effect: WaveModel or Particle Model• Notice that an electron cannot simply accumulate photons until it has enough energy. Only one photon interacts with one electron. The photon either has enough energy to eject the electron or it does not. Thus, the photon behaves more like a particle than a wave.• The photoelectric effect is perhaps the most direct and convincing evidence of the existence of photons and the "corpuscular" nature of light and electromagnetic radiation.• Albert Einstein received the Nobel Prize in physics in 1921 for explaining the photoelectric effect and for his contributions to theoretical physics.
• Unit 2 – The Nature of LightThe Photoelectric Effect: WaveModel or Particle Model
• Unit 2 – The Nature of LightThe Photoelectric Effect: WaveModel or Particle Model
• Unit 2 – The Nature of LightThe Principle ofComplementarity and Light• As we evaluate the predictions of the particle and wave models of light we find that each model explains some phenomenon and each model has difficulty explaining other phenomenon.• The photoelectric effect reveals that light has a particle nature. For properties such as reflection, refraction, diffraction and interference light behaves more like a wave.
• Unit 2 – The Nature of LightThe Principle ofComplementarity and Light• It has become obvious that light is not just a wave and not just a particle. It has a dual nature, a property referred to by physicists as a wave- particle duality. We can come to this conclusion because both theories of light have been shown to be valid based on very strong experimental evidence. It is clear that light is a much more complex phenomenon than just a beam of particles or just a simple wave.
• Unit 2 – The Nature of LightThe Principle ofComplementarity and Light• The Principle of Complementarity, proposed by Neils Bohr, stated that understanding both the wave and particle properties of light is essential if one is to have a full understanding of light. In other words, the two aspects of light complement one another.
• Unit 2 – The Nature of LightModern Theory of Light
• Unit 2 – The Nature of LightThe Speed of Light
• Unit 2 – The Nature of LightThe Speed of Light• Measuring the speed of light has always been a challenge.• Galileo was one of the first who attempted to calcualte the speed of light.• Then in the 1600s Roemer and Huygens used similar approaches by looking at the eclipses and orbits of Jupiter and one of its moons, Io.• Then in the 1800s Fizeau and Foucault used rotating wheels and mirrors to measure the speed of light.• In the 1900s, Michelson improved on Foucault’s technique and obtained a very accurate measurement of the speed of light.
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