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Particle Properties of Waves
LIGHTWAVES vs PARTICLE
The Corpuscular Theory• Newton: light consists of streams of tiny particles, which he called “corpuscles”.• Rectilinear Propagation• Reflection• Refraction
The Wave Theory• Christian Huygens : the wave nature of light was firmly established• Interference• Diffraction
Rectilinear PropagationWave fronts - The portions of water surface whose particles that are all in the same phase of motion. *The direction of propagation of the advancing straight wave is perpendicular to the wave front.
ReflectionA wave is turned back, or reflected, when itencounters a barrier that is boundary of themedium in which the wave is traveling.
Reflection I and r is 0 degree when the incident waveapproaches the barrier along a lineperpendicular to it.
ReflectionLaw of Reflection: i=rWhen a wave disturbance is reflected at theboundary of a transmitting medium, the angle ofincidence is equal to the angle of reflection.
RefractionThe bending of the path of a wave disturbanceas it passes obliquely from one medium intoanother of different propagation speed.
Refraction Water waves travel faster on the surface of deep water than they do on shallow water. The change in speed of the wave will cause refraction. The slower wave in the shallow water has a smaller wavelength.
DiffractionSpreading of a wave disturbance beyond theedge of a barrier.Set-up: Place two straight barriers across thetray on a line parallel with the straight waygenerator. An aperture, or opening, is leftbetween them approximately equal to thewavelength of the wave to be used. As asegment of each wave crest passes through theaperture, it clearly spreads into the regionbeyond the barriers.
DiffractionThe diffraction of a periodic straight wave as itpasses through a small aperture. Observe thedecrease in the diffraction effect as thewavelength of the disturbance sent against thebarrier is shortened.
The Superposition Principle When two or more waves travelsimultaneously through the same medium,(I) each wave proceeds independently asthough no other waves were present and(2) the resultant displacement of anyparticle is the vector sum of thedisplacements that the individual wavesacting alone would give it.
The Superposition Principle Y1- black solid line Y2- black dashed line Y- red lineIn effect, the displacement of any particle of the mediumby one wave at any instant is superimposed on thedisplacement of that particle by the other wave at thatinstant. The action of each wave on a particle isindependent of the action of the other, and the particledisplacement is the resultant of both wave action.
InterferenceThe general term interference is used todescribe the effects produced by two ormore waves that superpose while passingthrough a given region.
InterferenceConstructive Interference -suppose the displacement of a particular particle caused by one wave at any instant is in the same direction as that caused by the other wave. Then the total displacement of that particle at that instant is the sum of the separate displacements (superposition principle). The resultant displacement is greater than either wave would have caused separately.
InterferenceDestructive Interference -if the displacement effects of the two waves on the particle are in opposite directions, they tend to cancel one another. The resultant displacement of that particle at that instant is the difference of the two separate displacements and is in the direction of the larger (superposition principle). The resultant displacement is less than one of the waves would have caused separately.
InterferenceComplete destructive interferenceIf two such opposite displacements are equal inmagnitude, the resultant displacement is zero. Thedestructive interference is complete. The particle is notdisplaced at all but is in it’s equilibrium position at thatinstant.
Electromagnetic Waves A periodic disturbance involvingelectric and magnetic force. They are all thesame kind of wavy disturbance that repeatsitself over a distance called the wavelength.
Electromagnetic wavesThe ELECTROMAGNETIC SPECTRUM is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is thecharacteristic distribution of electromagnetic radiationemitted or absorbed by that particular object.
The Photoelectric Effect The emission of electrons by a substance when illuminated by electromagnetic radiation.
The Photoelectric EffectThe photoelectric effect was accidentallydiscovered by Heinrich Hertz in 1887during the course of the experiment thatdiscovered radio waves.Observation: when a negatively chargedbody was illuminated with light, its chargewas diminished.
The Photoelectric EffectJ.J. Thomson and P. Lenard determined theratio e/m for the particles emitted by the bodyunder illumination – the same as for electrons.The effect remained unexplained until 1905when Albert Einstein postulated the existenceof quanta of light -- photons -- which, whenabsorbed by an electron near the surface of amaterial, could give the electron enough energyto escape from the material.
The Photoelectric EffectRobert Milliken carried out a careful setof experiments, extending over ten years,that verified the predictions of Einstein’sphoton theory of light.
The Photoelectric Effect
The Photoelectric EffectObservations:• For a given material of the cathode, the “stopping” voltage does not depend on the light intensity – the energy of photons is determined by the light frequency, not intensity• The saturation current is proportional to the intensity of light at f =const – the saturation current is proportional to the number of photons, thus to the light intensity• Material-specific “red boundary” f0 exists: no photocurrent at f < f0 – at f < f0 (hf < W) the photon energy is insufficient to extract an electron from metal
The Photoelectric Effect It takes a certain amount of energy for an electron to escape from the metal. Electrons absorb this energy from the light Light is made up of photons with a certain amount of energy given by E = hfh = planck’s constant (6.63x10^-34)f = frequency
The Photoelectric EffectEnergy of the photon goes into:1. work function – work to free the electron2. kinetic energy of the electron
The Photoelectric EffectSample Problem: Radiation with a wavelength of 200 nm strikes a metal surface in a vacuum. Ejected electrons have a maximum speed of 7.22x10^5 m/s. What is the work function of the metal in eV?
The Photoelectric Effect Given: h (planck’s constant) = 6.63 x 10^-34 wavelength= 200 x 10^-9 m Speed (v) = 7.22 x 10^5 m/s m (mass of electron) = 9.1 x 10^-31 kg
The Photoelectric Effectf= speed of light wavelengthf = 3 x 10^8 m/s 200 x 10^-9 m = 1.5 x 10^15 Hz
The Photoelectric Effect(6.63 x 10^-34 J.s) 1.5 x 10^15 Hz = W + ½ (9.1 x 10^-31 kg)(7.22 x 10^5 m/s)^2 9.94 x10^-19 J = W + 2.37 x 10^-19 J (9.94 x 10^-19 J) – (2.37 x 10^-19 J) = W W = 7.57 x 10^ -19 J (7.57 x 10^-19 J) x 1ev = 4.73 eV 1.6 x 10^-19 J