Young/Freeman University Physics 11e

Ch 38 Photons, Electrons, and Atoms © 2005 Pearson Education

38.1 Emission and Absorption of Light Line spectra Line spectra Photoelectric Effect Photoelectric Effect X-Rays X-Rays Photons and Energy Levels Photons and Energy Levels © 2005 Pearson Education

38.1 Emission and Absorption of Light © 2005 Pearson Education Continuous spectrum Line spectrum

Hydrogen

Carbon

Sodium

38.2 Photoelectric Effect When light is incident on certain metallic surfaces, electrons are emitted from the surface When light is incident on certain metallic surfaces, electrons are emitted from the surface This is called the photoelectric effect This is called the photoelectric effect The emitted electrons are called photoelectrons The emitted electrons are called photoelectrons The effect was first discovered by Hertz The effect was first discovered by Hertz The successful explanation of the effect was given by Einstein in 1905 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 Received Nobel Prize in 1921 for paper on electromagnetic radiation, of which the photoelectric effect was a part

Photoelectric Effect Schematic When light strikes E, photoelectrons are emitted When light strikes E, photoelectrons are emitted Electrons collected at C and passing through the ammeter are a current in the circuit 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 C is maintained at a positive potential by the power supply

Photoelectric Current/Voltage Graph The current increases with intensity, but reaches a saturation level for large ΔV ’ s The current increases with intensity, but reaches a saturation level for large ΔV ’ s No current flows for voltages less than or equal to – ΔV s, the stopping potential No current flows for voltages less than or equal to – ΔV s, the stopping potential The stopping potential is independent of the radiation intensity The stopping potential is independent of the radiation intensity

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 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 is independent of the light intensity The maximum kinetic energy of the photoelectrons increases with increasing light frequency The maximum kinetic energy of the photoelectrons increases with increasing light frequency Electrons are emitted from the surface almost instantaneously, even at low intensities Electrons are emitted from the surface almost instantaneously, even at low intensities

Einstein ’ s Explanation Energy from the light beam is transferred to the electrons in the solid by photons which have an energy related to the freqency of the beam. Energy from the light beam is transferred to the electrons in the solid by photons which have an energy related to the freqency of the beam. The photon ’ s energy would be E = h ƒ The photon ’ s energy would be E = h ƒ Each photon can give all its energy to an electron in the metal Each photon can give all its energy to an electron in the metal The electron is considered to be in a well of height  which  is called the work function of the metal The electron is considered to be in a well of height  which  is called the work function of the metal Because of energy conservation the maximum kinetic energy of the liberated photoelectron is Because of energy conservation the maximum kinetic energy of the liberated photoelectron is KE = h ƒ – Φ

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 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 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 depends only on the frequency and the work function, not on the intensity The maximum KE increases with increasing frequency 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 The effect is instantaneous since there is a one-to-one interaction between the photon and the electron

38.3 Atomic Line Spectra and Energy Levels energy of emitted photon © 2005 Pearson Education

energy levels of the hydrogen atom © 2005 Pearson Education The hydrogen spectrum

© 2005 Pearson Education

38.5 The Bohr Model of an atom © 2005 Pearson Education Classical physics prediction Electron should continuously radiate electromagnetic waves and spiral into the nucleus

© 2005 Pearson Education Bohr model Proton is assumed to be stationary; the electron revolves in a circle of radius r n with speed v n

© 2005 Pearson Education In the Bohr model of the hydrogen atom, the permitted values of angular momentum are integral multiples of h/2π. The integer multiplier n is called the principal quantum number for the level. The orbital radii are proportional to n 2 and the orbital speeds are proportional to 1/n. (See Example 38.6)

© 2005 Pearson Education Energy level of different atoms

38.6 The Laser © 2005 Pearson Education

Population inversion

38.7 Production of X-rays X-rays are produced when high-speed electrons are suddenly slowed down X-rays are produced when high-speed electrons are suddenly slowed down Can be caused by the electron striking a metal target Can be caused by the electron striking a metal target A current in the filament causes electrons to be emitted A current in the filament causes electrons to be emitted These freed electrons are accelerated toward a dense metal target These freed electrons are accelerated toward a dense metal target

Production of X-rays An electron passes near a target nucleus An electron passes near a target nucleus The electron is deflected from its path by its attraction to the nucleus The electron is deflected from its path by its attraction to the nucleus This produces an acceleration This produces an acceleration It will emit electromagnetic radiation when it is accelerated It will emit electromagnetic radiation when it is accelerated The maximum x-ray energy, and minimum wavelength results when the electron loses all its energy in a single collision, such that e  V = hf max = hc/ min or therefore

© 2005 Pearson Education The laser operates on the principle of stimulated emission, by which many photons with identical wavelength and phase are emitted. Laser operation requires a non-equilibrium condition called a population inversion, in which more atoms are in a higher-energy state than are in a lower-energy state.

X-ray spectrum

38.8 The Compton Effect Compton directed a beam of x-rays toward a block of graphite Compton directed a beam of x-rays toward a block of graphite He found that the scattered x-rays had a slightly longer wavelength that the incident x-rays He found that the scattered x-rays had a slightly longer wavelength that the incident x-rays This means they also had less energy This means they also had less energy The amount of energy reduction depended on the angle at which the x-rays were scattered The amount of energy reduction depended on the angle at which the x-rays were scattered The change in wavelength is called the Compton shift The change in wavelength is called the Compton shift The Compton shift depends on the scattering angle and not on the wavelength The Compton shift depends on the scattering angle and not on the wavelength

Compton scattering © 2005 Pearson Education

38.8 Blackbody Radiation An object at any temperature is known to emit electromagnetic radiation An object at any temperature is known to emit electromagnetic radiation Sometimes called thermal radiation Sometimes called thermal radiation Stefan ’ s Law states that the total power radiated is given as Stefan ’ s Law states that the total power radiated is given as P =  T 4 P =  T 4 The wavelength λ m at which a blackbody radiates most strongly is inversely proportional to T. The Planck radiation law gives the spectral emittance I(λ) (intensity per wavelength interval in blackbody radiation.

The total radiated intensity (average power radiated per area) from a blackbody surface is proportional to the fourth power of the absolute temperature T. The quantity is called the Stefan- Boltzmann constant. The wavelength λ m at which a blackbody radiates most strongly is inversely proportional to T. The Planck radiation law gives the spectral emittance I(λ) (intensity per wavelength interval in blackbody radiation. © 2005 Pearson Education

END