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physics
modern physics

Questions and Answers of Modern Physics

In both the photoelectric effect and in the Compton Effect a photon collides with an electron causing the electron to fly off. What then, is the difference between the two processes?
Consider a point course of light. How would the intensity of light vary with distance from the source according to? (a) Wave theory(b) Particle (photon) theory? Would this help to distinguish the two
Explain how the photoelectric circuit of Fig. 27-6 could be used in (a) A burglar alarm,(b) A smoke detector. (c) A photographic light meter.
Why do we say that light has wave properties? Why do we say that light has particle properties?
Why do we say that electrons have wave properties? Why do we say that electrons have particle properties?
What is the difference between a photon and an electron? Be specific make a list.
If an electron and a proton travel at the same speed, which has the shorter wave length, Explain.
In Rutherford’s planetary model of the atom, what keeps the electron from flying off into space?
How can you tell if there is oxygen near the surface of the Sun?
When a wide spectrum of light passes through hydrogen gas at room temperature, absorption lines are observed that correspond only to the Lyman series. Why don’t we observe the other series?
Explain how the closely spaced energy levels for hydrogen near the top of Fig 27-27 correspond to the closely spaced spectral lines at the top of Fig. 27.22.
Is it possible for the de Broglie wavelength of a “particle” to be greater than the dimensions of the particle? To be smaller is there any direct connection?
In a helium atom, which contains two electrons, do you think that on average the electrons are closer to the nucleus or farther away than in a hydrogen atom? Why?
How can the spectrum of hydrogen contain so many lines when hydrogen contains only one electron?
The Lyman series is brighter than the Balmer series because this series of transitions ends up in the most common state for hydrogen, the ground state. Why then was the Balmer series discovered first?
Use conservation of momentum to explain why photons emitted by hydrogen atoms have slightly less energy than that predicted by Eq. 27-10.
The work functions for sodium and cesium are 2.28cV and 2.14eV, respectively. For incident photons of a given frequency, which metal will give a higher maximum kinetic energy for the electrons?
(a) Does a beam of infrared photons always have less energy than a beam of ultraviolet photons? Explain (b) Does a single infrared photon always have less energy than a single ultraviolet photon?
Light of 450-nm wavelength strikes a metal surface and a stream of electrons emerges from the metal. If light of the same intensity but of wavelength 400nm strikes the surface, are more electrons
Suppose we obtain an emission spectrum for hydrogen at very high temperature (when some of the atoms are in excited states), and an absorption spectrum at room temperature, when all atoms are in the
What is the value of e/m for a particle that moves in a circle of radius 7.0 mm in a 0.86-T magnetic field if a perpendicular 320-V/m electric field will make the path straight?
(a) What is the velocity of a beam of electrons that go undeflected when passing though crossed (perpendicular) electric and magnetic fields of magnitude 1.88 x 104 V/m and 2.90 x 10–3 T,
An oil drop whose mass is determined to be 2.8 x 10–15kg is held at rest between two large plates separated by 1.0 cm when the potential difference between them is 340 V. How many excess electrons
How hot is a metal being welded if it radiates most strongly at 440nm?
Estimate the peak wavelength for radiation from (a) Ice at 0oC(b) A food lamp at 3500K(c) Helium at 4 K.(d) For the universe at T = 2.725K, assuming black, body emission. In what region of the EM
(a) What is the temperature of the peak of a blackbody spectrum is at 18.0nm?(b) What is the wavelength at the peak of a blackbody spectrum if the body is at a temperature of 2000K?
An HCI molecule vibrates with a natural frequency of 8.1 x 1013 Hz. What is the difference in energy (in joules and electron volts) between possible values of the oscillation energy?
The steps of a flight of stairs are 20.0cm high (vertically). IF a 68.0-kg person stands with both feet on the same step, what is the gravitational potential energy of this person, relative to the
Estimate the peak wave length of light issuing from the pupil of the human eye (which approximates a black-body) assuming normal body temperature.
What is the energy of photons (joules) emitted by an 88.5-MHz FM radio station?
What is the energy range (in joules and eV) of photons in the visible spectrum, of wavelength 400nm to 750nm?
A typical gamma ray emitted from a nucleus during radioactive decay may have an energy of 300keV. What is its wavelength? Would we expect significant diffraction of this type of light when it passes
About 0.1eV is required to break “hydrogen bond” in a protein molecule. Calculate the minimum frequency and maximum wavelength of a photon that can accomplish this.
Calculate the momentum of a photon of yellow light of wavelength 6.00 x 10–7m.
What is the momentum of a λ = 0.010nm X-ray photon?
The human eye can respond to as little as 10–18 J of light energy. For a wavelength at the peak of visual sensitivity, 550nm, how many photons lead to an observable flash?
What minimum frequency of light is needed to eject electrons from a metal whose work function is 4.3 x 10–19J?
What is the longest wavelength of light that will emit electrons from a metal whose work function is 3.10eV?
The work functions for sodium, cesium, copper, and iron are 2.3, 2.1, 4.7 and 4.5 eV, respectively. Which of those metals will not emit electrons when visible light shines on it?
In a photoelectric-effect experiment it is observed that no current flows unless the wavelength is less than 570nm.(a) What is the work function of this material?(b) What is the stopping voltages
What is the maximum kinetic energy of electrons ejected from barium (Wo = 2.48eV) when illuminated by white light, λ = 400 to 750nm?
Barium has a work function of 2.48eV. What is the maximum kinetic energy of electrons if the metal is illuminated by UV light of wavelength 365nm? What is their speed?
When UV light of wavelength 285nm falls on a metal surface, the maximum kinetic energy of emitted electrons is 1.40eV. What is the work function of the metal?
The threshold wavelength for emission of electrons from a given surface is 350nm. What will be the maximum kinetic energy of ejected electrons when the wavelength is changed to (a) 280nm,(b) 360nm?
A certain type of film is sensitive only to light whose wavelength is less than 660 nm. What is the energy (eV and kcal/mol) needed for the chemical reaction to occur which causes the film to change?
When 230-nm light falls on a metal the current through a photoelectric circuit (Fig 27-6) is brought to zero at a stopping voltage of 1.64V. What is the work function of the metal?
In a photoelectric experiment using a clean sodium surface, the maximum energy of the emitted photons was measured for a number of different incident frequencies, with the following resultsPlot the
Show that the energy E (in electron volts) of a photon whose wavelength is λ (nm) is given by
The quantity h/m0c, which has the dimensions of length, is called the Compton wavelength for(a) An electron, (b) A proton.(c) Show that if a photon has wavelength equal to the Compton wavelength of a
X-rays of wavelength λ = 0.120 nm are scattered from carbon. What is the Compton wavelength shift for photons detected at angles (relative to the incident beam) of? (a) 45o (b) 90o (c) 180o?
In the Compton Effect, a 0.100-nm photon strikes a free electron in a head-on-collision and knocks it into the forward direction. The rebounding photon recoils directly backward. Use conservation of
How much total kinetic energy will an electron-positron pair have if produced by a 3.84-MeV photon?
What is the longest wavelength photon that could produce a proto-antiproton pair? (Each has a mass of 1.67 x 10–27kg.)
What is the minimum photon energy needed to produce a μ – μ pair? The mass of each μ (muon) is 207 times the mass of the electron. What is the wavelength of such a photon?
An electron and a positron, each moving at 1.0 x 105m/s, collide head on, disappear, and produce two photons, each with the same energy and momentum moving in opposite direction. What is the energy
A gamma-ray photon produces an electron-positron pair, each with a kinetic energy of 245keV. What was the energy and wavelength of the photon?
Calculate the wavelength of a 0.23-kg ball traveling at 0.10m/s.
What is the wavelength of a neutron (m = 1.67 x 10–27kg) traveling at 6.5 x 104m/s?
Through how many volts of potential difference must an electron be accelerated to achieve a wavelength of 0.24nm?
Calculate the ratio of the kinetic energy of an electron to that of a proton if their wavelengths are equal. Assume that the speeds are non-relativistic.
An electron has a de Broglie wavelength λ = 5.0 x 10–10m. (a) What is its momentum? (b) What is its speed? (c) What voltage was needed to accelerate it to this speed?
What is the wavelength of an electron of energy? (a) 10eV,(b) 100eV(c) 1.0keV?
Show that if an electron and a proton have the same non-relativistic kinetic energy, the proton has the shorter wavelength.
Calculate the de Broglie wavelength of a electron in your TV picture tub if it is accelerated by 30,000V. Is it relativistic? How does its wavelength compare to the size of the “neck” of the
Ferrari with a mass of 1400 kg approaches a freeway underpass that is 10m across. At what speed must the car be moving, in order for it to have wavelength such that it might somehow “diffract”
What voltage is needed to produce electron wavelengths of 0.20nm? (Assume that the electron is non-relativistic).
Electrons are accelerated by 2450 V in an electron microscope. What is the maximum possible resolution?
For the three hydrogen transitions indicated below, with n being the initial state and n’ being the final state, is the transition an absorption or an emission? Which is higher, the initial state
How much energy is needed to ionize a hydrogen atom in the n = 2 state?
The third longest wavelength in the Paschen series in hydrogen (Fig. 27-27) corresponds to what transition?
Calculate the ionization energy of doubly ionized lithium, Li2+, which has Z = 3.
(a) Determine the wavelength of the second Balmer line (n = 4 to n = 2 transition) using Fig. 27-27. Determine likewise (b) The wavelength of the second Lyman line and(c) The wavelength of the third
Evaluate the Rydberg constant R using Bohr theory (compare Eqs. 27-9 and 27-16) and show that its value is R = 1.0974 x 10 7 m–1.
What is the longest wavelength light capable of ionizing a hydrogen atom in the ground state?
What wavelength photon would the required to ionize a hydrogen atom in the ground state and given the ejected electron a kinetic energy of 10.0eV?
In the Sun, an ionized helium (He+) atom makes a transition from the n = 6 state to the n = 2 state emitting a photon. Can that photon be absorbed by hydrogen atoms present in the Sun? If so, between
Construct the energy-level diagram for the He+ ion (See Fig 27-27).
Construct the energy-level diagram for doubly ionized lithium, Li2+.
What is the potential energy and the kinetic energy of an electron in the ground state of the hydrogen atom?
An excited hydrogen atom could, in principle, have a radius of 1.00mm. What would be the value of n for a Bohr orbit of this size? What would its energy be?
Is the use of non-relativistic formulas justified in the Bohr atom? To check, calculate the electron’s velocity v, in terms of c, for the ground state of hydrogen, and then calculate √1 –
Suppose an electron was bound to a proton, as in the hydrogen atom, but by the gravitational force rather than by the electric force, what would be the radius and energy of the first Bohr orbit?
Suppose a particle of mass m is confined to a one-dimensional box of width L. According to quantum theory the particle’s wave (with λ = h/mv) is a standing wave with nodes at the edges of the
The Big Bang theory states that the beginning of the universe was accompanied by a huge burst of photons. Those photons are still present today and make up the so-called cosmic microwave background
At low temperatures, nearly all the atoms in hydrogen gas will be in the ground state. What minimum frequency photon is needed if the photoelectric effect is to be observed?
A beam of 85-eV electrons is scattered from a crystal, as in X-ray diffraction, and a first-order peak is observed at θ = 38o. What is the spacing between planes in the diffracting crystal?
A microwave oven produces electromagnetic radiation at λ = 12.12cm and produces a power of 760 W. Calculate the number of microwave photons produced by the microwave oven each second.
Sunlight reaching the Earth’s surface has an intensity of about 1000W/m2. Estimate how many photons per square meter per second this represents. Take the average wavelength of be 550nm.
A beam of red laser light (λ = 633nm) hits a black wall and is fully absorbed. If this light exerts a total force F = 5.5nN on the wall, how many photons per second are hitting the wall?
If a 100-W lightbulb emits 3.0% of the input energy as visible light (average wavelength 550nm) uniformly in all directions, estimate how many photons per second of visible light will strike the
An electron and a positron collide head on, annihilate, and create two 0.90-MeV photons traveling in opposite directions. What were the initial kinetic energies of electron and positron?
By what potential difference must (a) A proton (m = 1.67 x 10–27 kg) and (b) An electron (m = 9.11 x 10– 31 kg), be accelerated to have a wavelength λ = 5.0 x 10 –12m?
In some of Rutherford’s experiment (Fig. 27-18) the a particle (mass = 6.64 x 10–27 kg) had a kinetic energy of 4.8 MeV. How close could they get to a gold nucleus (charge = + 79e)? Ignore the
By what fraction does the mass of an H atom decrease when it makes an n = 3 to n = 1 transition?
Calculate the ratio of the gravitational to electric force for the electron in a hydrogen atom. Can the gravitational force be safely ignored?
Electrons accelerated by a potential difference of 12.2V pass through a gas of hydrogen atoms at room temperature. What wavelengths of light will be emitted?
In a particular photoelectric experiment, a stopping potential of 2.10V is measured when ultraviolet light of wavelength 290nm is incident on the metal. Using the same setup, what will the new
In an X-ray tube (see Fig. 25-35 and discussion in Section 25-11), the high voltage between filament and target is V. After being accelerated through this voltage and electron strikes the target
The intensity of the Sun’s light in the vicinity of the Earth is about 1000 W/m2. Imagine a spacecraft with a mirrored square sail of dimension 1.0km. Estimate how much thrust (in newtons) this
Light of wavelength 300 nm strikes a metal whose work function is 2.2eV. What is the shortest de Broglie wavelength for the electrons that are produced as photoelectrons?

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