Physics 1161: Lecture 22

Transcription

1 Physics 1161: Lecture 22 Blackbody Radiation Photoelectric Effect Wave-Particle Duality sections Everything comes unglued The predictions of classical physics (Newton s laws and Maxwell s equations) are sometimes WRONG. classical physics says that an atom s electrons should fall into the nucleus and STAY THERE. No chemistry, no biology can happen. classical physics says that toaster coils radiate an infinite amount of energy: radio waves, visible light, X-rays, gamma rays, The source of the problem It s not possible, even in theory to know everything about a physical system. knowing the approximate position of a particle corrupts our ability to know its precise velocity ( Heisenberg uncertainty principle ) Particles exhibit wave-like properties. interference effects! Quantum Mechanics! At very small sizes the world is VERY different! Energy can come in discrete packets Everything is probability; very little is absolutely certain. Particles can seem to be in two places at same time. Looking at something changes how it behaves. Blackbody Radiation Blackbody Radiation Spectrum Hot objects glow (toaster coils, light bulbs, the sun). Visible Light: ~0.4µm to 0.7µm As the temperature increases the color shifts from Red to Blue. The classical physics prediction was completely wrong! (It said that an infinite amount of energy should be radiated by an object at finite temperature.) Higher temperature: peak intensity at shorter λ 1

2 Blackbody Radiation: First evidence for Q.M. Max Planck found he could explain these curves if he assumed that electromagnetic energy was radiated in discrete chunks, rather than continuously. The quanta of electromagnetic energy is called the photon. Energy carried by a single photon is E= hf= hc/λ Planck s constant: h= X Joule sec Preflights 22.1, 22.3 A series of light bulbs are colored red, yellow, and blue. Which bulb emits photons with the most energy? The least energy? Which is hotter? (1) stove burner glowing red (2) stove burner glowing orange Preflights 22.1, 22.3 A series of light bulbs are colored red, yellow, and blue. Which bulb emits photons with the most energy? Blue! Lowest wavelength is highest energy. The least energy? Which is hotter? E = hf= hc/λ Red! Highest wavelength is lowest energy. (1) stove burner glowing red (2) stove burner glowing orange Hotter stove emits higher-energy photons (shorter wavelength = orange) Three light bulbs with identical filaments are manufactured with different colored glass envelopes: one is red, one is green, one is blue. When the bulbs are turned on, which bulb s filament is hottest? λ 1. Red max 2. Green 3. Blue 4. Same A redand greenlaser are each rated at 2.5mW. Which one produces more photons/second? 1. Red 2. Green 3. Same Wien s Displacement Law To calculate the peak wavelength produced at any particular temperature, use Wien s Displacement Law: T λ peak = *10-2 m K 0% 0% 0%

3 For which work did Einstein receive the Nobel Prize? 1. Special Relativity E = mc 2 2. General Relativity Gravity bends Light 3. Photoelectric Effect Photons 4. Einstein didn t receive a Nobel prize. Photoelectric Effect Light shining on a metal can knock electrons out of atoms. Light must provide energy to overcome Coulomb attraction of electron to nucleus Light Intensity gives power/area (i.e. Watts/m 2 ) Recall: Power = Energy/time (i.e. Joules/sec.) Photoelectric Effect Light Intensity Threshold Frequency Glass is not transparent to ultraviolet light Light in visible region is lower frequency than ultraviolet There is a minimum frequency necessary to eject electrons Difficulties With Wave Explanation effect easy to observe with violet or ultraviolet (high frequency) light but not with red (low frequency) light rate at which electrons ejected proportional to brightness of light The maximum energy of ejected electrons NOT affected by brightness of light electron's energy depends on light s frequency 3

4 Photoelectric Effect Summary Each metalhas Work Function (W 0 )which is the minimum energy needed to free electron from atom. Light comes in packets called Photons E = h f h=6.626 X Joule sec Maximum kinetic energy of released electrons hf= KE + W 0 If hffor the light incident on a metal is equal to the work function, what will the kinetic energy of the ejected electron be? 1. the kinetic energy would be negative 2. the kinetic energy would be zero 3. the kinetic energy would be positive 4. no electrons would be released from the metal If hffor the light incident on a metal is less than the work function, what will the kinetic energy of the ejected electron be? 1. the kinetic energy would be negative 2. the kinetic energy would be zero 3. the kinetic energy would be positive 4. no electrons would be released from the metal Photoelectric: summary table Wave Increase Intensity Particle Result Rate Increase Increase Increase KE Increase Unchanged Unchanged Increase Frequency Rate Unchanged Increase Increase KE Unchanged Increase Increase Light is composed of particles: photons Preflights 22.4, 22.6 Which drawing of the atom is more correct? Is Light a Wave or a Particle? Wave Electric and Magnetic fields act like waves Superposition, Interference and Diffraction This is a drawing of an electron s p-orbital probability distribution. At which location is the electron most likely to exist? Particle Photons Collision with electrons in photo-electric effect Both Particle and Wave! 4

5 11/15/2010 Are Electrons Particles or Waves? Particles, definitely particles. You can see them. You can bounce things off them. You can put them on an electroscope. How would know if electron was a wave? The approximate numbers of photons at each stage are (a) 3 103, (b) , (c) , (d) , (e) , and (f) Interference Pattern Develops Stages of two-slit interference pattern. The pattern of individually exposed grains progresses from (a) 28 photons to (b) 1000 photons to (c) 10,000 photons. As more photons hit the screen, a pattern of interference fringes appears. How Do They Know photons hit the film at places they would not hit if both slits were open! If we think about this classically, we are perplexed and may ask how photons passing through the single slit know that the other slit is covered and therefore fan out to produce the wide single-slit diffraction pattern. Look for interference! Single Slit Diffraction If we cover one slit so that photons hitting the photographic film can only pass through a single slit, the tiny spots on the film accumulate to form a single-slit diffraction pattern How Do They Know? Or, if both slits are open, how do photons traveling through one slit know that the other slit is open and avoid certain regions, proceeding only to areas that will ultimately fill to form the fringed double-slit interference pattern? 5

6 Modern Answer modern answer is that the wave nature of light is not some average property that shows up only when many photons act together Each single photon has wave as well as particle properties. But the photon displays different aspects at different times. Wavicle? photon behaves as a particle when it is being emitted by an atom or absorbed by photographic film or other detectors photon behaves as a wave in traveling from a source to the place where it is detected photon strikes the film as a particle but travels to its position as a wave that interferes constructively Electrons? fact that light exhibits both wave and particle behavior was one of the interesting surprises of the early twentieth century. even more surprising was the discovery that objects with mass also exhibit a dual waveparticlebehavior Electrons are Waves? Electrons produce interference pattern just like light waves. Need electrons to go through both slits. What if we send 1 electron at a time? Does a single electron go through both slits? Electrons are Particles and Waves! Depending on the experiment electron can behave like wave (interference) particle (localized mass and charge) If we don t look, electron goes through both slits. If we do look it chooses 1. Electrons are Particles and Waves! Depending on the experiment electron can behave like wave (interference) particle (localized mass and charge) If we don t look, electron goes through both slits. If we do look it chooses 1 of them. 6

7 Quantum Summary Particles act as waves and waves act as particles Physics is NOT deterministic Observations affect the experiment 7