What is Franck-hertz: Definition and 16 Discussions

The Franck–Hertz experiment was the first electrical measurement to clearly show the quantum nature of atoms, and thus "transformed our understanding of the world". It was presented on April 24, 1914, to the German Physical Society in a paper by James Franck and Gustav Hertz. Franck and Hertz had designed a vacuum tube for studying energetic electrons that flew through a thin vapor of mercury atoms. They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy before flying away. This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.
These experimental results proved to be consistent with the Bohr model for atoms that had been proposed the previous year by Niels Bohr. The Bohr model was a precursor of quantum mechanics and of the electron shell model of atoms. Its key feature was that an electron inside an atom occupies one of the atom's "quantum energy levels". Before the collision, an electron inside the mercury atom occupies its lowest available energy level. After the collision, the electron inside occupies a higher energy level with 4.9 electron volts (eV) more energy. This means that the electron is more loosely bound to the mercury atom. There were no intermediate levels or possibilities in Bohr's quantum model. This feature was "revolutionary" because it was inconsistent with the expectation that an electron could be bound to an atom's nucleus by any amount of energy.In a second paper presented in May 1914, Franck and Hertz reported on the light emission by the mercury atoms that had absorbed energy from collisions. They showed that the wavelength of this ultraviolet light corresponded exactly to the 4.9 eV of energy that the flying electron had lost. The relationship of energy and wavelength had also been predicted by Bohr. After a presentation of these results by Franck a few years later, Albert Einstein is said to have remarked, "It's so lovely it makes you cry."On December 10, 1926, Franck and Hertz were awarded the 1925 Nobel Prize in Physics "for their discovery of the laws governing the impact of an electron upon an atom".

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  1. Terrycho

    Why is the first peak in the Franck-Hertz experiment longer than the others?

    In the experiment, I know that the spacing between successive valleys gives the excitation energy to be somewhere around 4.9eV. However, when you look at the plot, you can see that the spacing from zero to the first peak is much longer than any other spacings between two successive peaks. I was...
  2. K

    Why Do Voltage-Current Graphs in the Franck-Hertz Experiment Show Tops and Dips?

    Homework Statement I have two questions concerning the Franck-Hertz experiment on Mercury. The first one is to explain the tops and dips in the voltage-current graph (https://en.wikipedia.org/wiki/File:Franck-Hertz_en.svg). The second question consider the experimental setup, and why there is...
  3. G

    I Why does a mercury Franck-Hertz tube produce visible light?

    Hi. I've recently conducted a Franck-Hertz experiment with mercury. I was able to see bluish glowing regions just as here: However, theory predicts 254 nm, which is far below visible. Are there other energy levels at play here? Wouldn't that mess up the 4.9-V-spaces of the drops in the current...
  4. G

    I Franck-Hertz: Overall behaviour of the current

    Hi. The blue curve shows the idealized behaviour of the detected current vs. the acceleration voltage in a Franck-Hertz experiment: It appears that the underlying behaviour is quadratic, why? I calculated the time for an electron to reach the grid at distance ##d## from the hot cathode to be...
  5. B

    How do you work out the wavelegth (Franck-Hertz experiment)?

    Homework Statement In a Franck-Hertz experiment carried out with potassium vapour, it is found that the current falls off rapidly at an applied voltage of 1.62V. Calculate the wavelength of the expected spectral line in the emission spectrum of potassium when this voltage is reached. Homework...
  6. Wrichik Basu

    B Question in Franck and Hertz Experiment

    My book gives the following graph for current vs accelerating potential for Franck and Hertz experiment (used to prove existence of discrete energy levels in atoms) using Mercury vapours in the tube: The book then writes: "Actually, atoms have more than one excitation potential and also an...
  7. H

    I Understanding the Franck-Hertz Experiment: Causes of Current Drop

    Hi In the FH experiment, what is physically happening when the excited mercury atom hits the metal plate? why does it induce a current drop? Thank you for any help :)
  8. I

    Contact potential difference

    Homework Statement Hi! I'm writing my lab report on the Franck-Hertz experiment and I have trouble with the contact potential difference (inelastic collision). How do you calculate it? Homework Equations See below The Attempt at a Solution The energy differences between the maximum and...
  9. T

    Why are the peaks in the Franck-Hertz experiment broad?

    As opposed to discrete values, given that the energy levels are discrete. The gas in mind here is helium. When studying the peaks a plot of accelerating voltage vs current that match up to the energy levels they should be discrete, right?
  10. ghaleb hamdan

    Does voltage affect the current in the Franck-Hertz experiment?

    I have a question that might be very basic. The Franck-Hertz experiment shows that as the voltage (KE of the electrons) increase, the current/energy of the electron increases up to 4.9v at which point it drops due to the non elastic collision. this happens at intervals of 4.9v. assume that the...
  11. P

    Franck-Hertz Experiment,interpretation of graph

    Homework Statement As it is shown in the attached file,there is multiple maximums of the function. i understand that the first one (about 4,9V) corresponds to resonant line of the atom. What I don't understand is why the next one is higher than the first one, what does it mean...
  12. A

    Temperature relevance of the Franck-Hertz Experiment

    I have to do a lab next week in which I repeat the Franck-Hertz experiment. I am reasonably sure that I understand most of what I need to do, but I'm just a little unsure on a couple of points, and would appreciate some guidance if anyone is knowledgeable with it. The instructions say that...
  13. A

    Help with a concrete question about the Franck-Hertz experiment

    I'm studying the Franck-Hertz experiment for my Quantum Physics class, and I don't see why the minimums for the graphic U_1 (accelerating voltage) vs I_a (collector intensity) increase. It seems logical, but I can't find a proper explanation. If anyone could help me it would be great :)...
  14. D

    How Do You Determine Quantum Numbers from Excitation Potentials?

    Homework Statement I've been given 2 values for the potential, 10.2 and 12.1V, I have to find the principle quantum numbers of which have been excited. The Attempt at a Solution I know En= -13.6/n^2 From there I'm stuck, I know it should go up in 4.9v, but 10.2 and 12.1 aren't multiples...
  15. R

    Solving Franck-Hertz Curve Questions for Mercury

    Homework Statement I've just done a practical to plot a franck-hertz curve for mercury. There are a few post-practical questions, and I'm not sure of 2 of them: 1) estimate the contact potential 2) estimate the mean free path of an electron in mercury vapour at a temperature of 160C and...
  16. Z

    Franck-Hertz (my head) Experiment

    My questions are about the classic Franck-Hertz experiment where electrons are given energy by an accelerating voltage. These collide with mercury atoms, raising their internal energy. The electrons are then left with insufficient energy to overcome the retarding voltage of a collector plate...