1. General Physics (PHY 2140) Lecture 28 Modern Physics Quantum Physics
Photoelectric effect
Chapter 27
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2. If you want to know your progress so far, please send me an request at
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3. Lightning Review Last lecture: Quantum physics Blackbody radiation
Planck’s hypothesis
Review Problem: All objects radiate energy. Why, then, are we not able to see all objects in a dark room?
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4. Reminder (for those who don’t read syllabus)
Reading Quizzes (bonus 5%):
It is important for you to come to class prepared, i.e. be familiar with the
material to be presented. To test your preparedness, a simple five-minute
quiz, testing your qualitative familiarity with the material to be discussed in
class, will be given at the beginning of some of the classes. No make-up
reading quizzes will be given.
There could be one today…
… but then again…
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5. QUICK QUIZ
A photon (quantum of light) is reflected from a mirror. True or false: (a) Because a photon has a zero mass, it does not exert a force on the mirror. (b) Although the photon has energy, it cannot transfer any energy to the surface because it has zero mass. (c) The photon carries momentum, and when it reflects off the mirror, it undergoes a change in momentum and exerts a force on the mirror. (d) Although the photon carries momentum, its change in momentum is zero when it reflects from the mirror, so it cannot exert a force on the mirror.
False
True
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6. Problem: a lightbulb
Assuming that the tungsten filament of a lightbulb is a blackbody, determine its peak wavelength if its temperature is K.
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7. This is infrared, so most of the electric energy goes into heat!
Assuming that the tungsten filament of a lightbulb is a blackbody, determine its peak wavelength if its temperature is K.
Recall Wein;s law: λmax T = x 10-2 m • K. It can be used to solve for lmax:
Given:
T = 2900 K
Find:
lmax = ?
Thus,
This is infrared, so most of the electric energy goes into heat!
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8. 27.2 Photoelectric Effect
When light is incident on certain metallic surfaces, electrons are emitted from the surface
This is called the photoelectric effect
The emitted electrons are called photoelectrons
The effect was first discovered by Hertz
The successful explanation of the effect was given by Einstein in 1905
Received Nobel Prize in 1921 for paper on electromagnetic radiation, of which the photoelectric effect was a part
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9. Photoelectric Effect Schematic
When light strikes E, photoelectrons are emitted
Electrons collected at C and passing through the ammeter are a current in the circuit
C is maintained at a positive potential by the power supply
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10. Photoelectric Current/Voltage Graph
The current increases with intensity, but reaches a saturation level for large ΔV’s
No current flows for voltages less than or equal to –ΔVs, the stopping potential
The stopping potential is independent of the radiation intensity
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11. Features Not Explained by Classical Physics/Wave Theory
No electrons are emitted if the incident light frequency is below some cutoff frequency that is characteristic of the material being illuminated
The maximum kinetic energy of the photoelectrons is independent of the light intensity
The maximum kinetic energy of the photoelectrons increases with increasing light frequency
Electrons are emitted from the surface almost instantaneously, even at low intensities
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12. Einstein’s Explanation
A tiny packet of light energy, called a photon, would be emitted when a quantized oscillator jumped from one energy level to the next lower one
Extended Planck’s idea of quantization to electromagnetic radiation
The photon’s energy would be E = hƒ
Each photon can give all its energy to an electron in the metal
The maximum kinetic energy of the liberated photoelectron is
KE = hƒ – Φ
Φ is called the work function of the metal
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13. Explanation of Classical “Problems”
The effect is not observed below a certain cutoff frequency since the photon energy must be greater than or equal to the work function
Without this, electrons are not emitted, regardless of the intensity of the light
The maximum KE depends only on the frequency and the work function, not on the intensity
The maximum KE increases with increasing frequency
The effect is instantaneous since there is a one-to-one interaction between the photon and the electron
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14. Verification of Einstein’s Theory
Experimental observations of a linear relationship between KE and frequency confirm Einstein’s theory
The x-intercept is the cutoff frequency
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15. Problem: photoeffect
Electrons are ejected from a metallic surface with speeds ranging up to 4.6 × 105 m/s when light with a wavelength of λ = 625 nm is used.
(a) What is the work function of the surface?
(b) What is the cutoff frequency for this surface?
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16. (a) What is the work function of the surface?
Electrons are ejected from a metallic surface with speeds ranging up to 4.6 × 105 m/s when light with a wavelength of λ = 625 nm is used.
(a) What is the work function of the surface?
(b) What is the cutoff frequency for this surface?
Recall that KEmax=hf - f. This can be used to solve for f. First find the kinetic energy
Given:
v = 4.6x105 m/s
l = 625 nm
Find:
f = ?
n =?
Thus,
It equals to 1.4 eV
Cutoff frequency is
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17. 27.3 Application: Photocells
Photocells are an application of the photoelectric effect
When light of sufficiently high frequency falls on the cell, a current is produced
Examples
Streetlights, garage door openers, elevators
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18. 27.4 X-Rays Electromagnetic radiation with short wavelengths
Wavelengths less than for ultraviolet
Wavelengths are typically about 0.1 nm
X-rays have the ability to penetrate most materials with relative ease
Discovered and named by Roentgen in 1895
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19. Production of X-rays
X-rays are produced when high-speed electrons are suddenly slowed down
Can be caused by the electron striking a metal target
A current in the filament causes electrons to be emitted
These freed electrons are accelerated toward a dense metal target
The target is held at a higher potential than the filament
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20. Production of X-rays An electron passes near a target nucleus
The electron is deflected from its path by its attraction to the nucleus
This produces an acceleration
It will emit electromagnetic radiation when it is accelerated
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21. Diffraction of X-rays by Crystals
For diffraction to occur, the spacing between the lines must be approximately equal to the wavelength of the radiation to be measured
For X-rays, the regular array of atoms in a crystal can act as a three-dimensional grating for diffracting X-rays
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22. Schematic for X-ray Diffraction
A continuous beam of X-rays is incident on the crystal
The diffracted radiation is very intense in certain directions
These directions correspond to constructive interference from waves reflected from the layers of the crystal
The diffraction pattern is detected by photographic film
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23. Photo of X-ray Diffraction Pattern
The array of spots is called a Laue pattern
The crystal structure is determined by analyzing the positions and intensities of the various spots
This is for NaCl
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24. Bragg’s Law
The beam reflected from the lower surface travels farther than the one reflected from the upper surface
If the path difference equals some integral multiple of the wavelength, constructive interference occurs
Bragg’s Law gives the conditions for constructive interference
2 d sin θ = m λ, m = 1, 2, 3…
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25. If you want to know your progress so far, please send me an request at
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