What Does the Franck Hertz Experiment Prove in Quantum Physics?
Franck Hertz experiment produced exceptional results which were fundamental in quantum theory development. After the presentation of Bohr’s theory of the atom, Franck Hertz’s experiment was undertaken in the year 1914 by James Franck and Gustav Hertz. It showed one of the initial symptoms of atoms having different levels of energy. The electrons in this test are passed and given a boost by mercury vapour. As a result, loss of energy takes place because of inelastic scattering.
Insight to the Franck Hertz Experiment Theory
In this experiment, a thermionic emission made by a filament produces an electron beam. First, electrons are powered, and then they pass through vapour and finally decelerate before anode. All these occur within a tube inside a burner that helps in controlling the temperature of the tube. Plus, it also controls the density of mercury vapour. The following diagram is the setup of the Franck Hertz tube along with electrical connections.
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When the filament is heated, the metal’s conduction electron energy also gets raised. With enhancing temperature, a substantial section of electrons experiences an increase in their kinetic energy more than the cathode material’s work function (here platinum). However, few of them get close to the anode, and a small amount of current flow takes place.
Further, when Va (accelerating voltage) is amplified (4.9 volts), filament electrons are pushed closer to the grid, and a significant amount of current touch the anode. If Vg becomes zero, a few of the electrons that are emitted, reach the anode and generates an electron current. The current path in this set-up propagates across the filament, power supplies, ammeter, and mercury vapour. Note that only some electrons finally reach the anode. With an increase in Va, the current in the anode also increases as the trajectories of electrons stay focused. Plus, less deflection of electron takes place due to scattering.
Next, consider collisions of mercury atoms with electrons. One reason for electron deflection is due to elastic scattering. In this event, an atom “recoils” like a solid sphere, restricting greater loss of energy. Moreover, inelastic scattering can also take place. Electrons of mercury are unable to receive energy unless the same touches the threshold. Note that 4.9 volts is similar to a line in the mercury spectrum at 254 nm. Moreover, drops occur several times in 4.9 volts in the current. The reason being if a raised electron holding energy of 4.9 eV is discarded during a collision, it can be powered again to yield a similar type of collision at 4.9 volts.
Frank and Hertz experiment: Information about the Apparatus
The Franck-Hertz tube is inserted in an oven – a box made of metal consisting of terminals and a thermostatically controllable heater for making connections. Plus, a thermometer can be put through the opening on the upper part of the apparatus.
This system has electrodes placed parallel to each other for generating a uniform electric field. Besides, space between the perforated grip and cathode is significantly high as compared to the average free-electron path across mercury vapour under normal working conditions. To identify a directly heated cathode in this tube, find a ribbon made of platinum alongside a barium-oxide spot. Plus, cathode limits are provided with an electrode that restricts the flow of current and diminishes reflected and secondary electrons. This provides a uniform electric field.
However, to prevent leakage, a ceramic lining is fused inside the glass. Furthermore, evacuation of the tube is done, and the inner part is coated with ‘getter’. This helps to absorb air that leaked out at the time of manufacturing and throughout the apparatus’s lifetime.
Reading of the anode current must be conducted with a picoammeter or Keithley model 610 Electrometer, while the rest of the voltages must be measured with digital voltmeters.
Franck Hertz Experiment Conclusion
Franck Hertz Experiment Data for Mercury
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The above image shows that electrons lose 4.9 eV with every collision they make with mercury atoms. One can observe ten sequential bumps after an interval of 4.9 volts.
Franck Hertz Experiment Data for Neon
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For neon gas, the powered electrons excite neon electrons to higher states and further decelerate in a way to provide a glow in the gas region. About ten excited levels can be acquired within the range of 18.3 to 19.5 eV, and they decelerate at a range of 16.57 to 16.79 eV. This difference in energy produces light.
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FAQs on Franck Hertz Experiment Explained: Theory, Setup & Significance
1. What was the main purpose of the Franck-Hertz experiment?
The main purpose of the Franck-Hertz experiment was to provide direct experimental proof for Niels Bohr's model of the atom. It aimed to show that atoms have discrete, quantized energy levels and can only absorb energy in specific, fixed amounts, not in a continuous spectrum.
2. Can you explain the basic setup used in the Franck-Hertz experiment?
The experiment uses a sealed glass tube containing mercury vapour at low pressure. Inside, it has:
- A cathode that is heated to emit electrons.
- An accelerating grid with a variable positive voltage that speeds up the electrons towards it.
- A collecting plate with a small opposing voltage that measures the current of electrons that pass the grid.
3. What do the peaks and dips in the Franck-Hertz experiment graph represent?
The graph plots the electric current against the accelerating voltage. The peaks show that electrons are passing through the mercury vapour without losing much energy. The sudden dips in current occur at specific voltages (multiples of 4.9V for mercury) where electrons undergo inelastic collisions with mercury atoms, lose energy, and fail to reach the collecting plate.
4. How does this experiment provide evidence for quantized energy levels?
The experiment shows that mercury atoms only absorb a precise amount of energy—4.9 electron volts (eV)—from colliding electrons. This energy corresponds to the first excitation level of mercury. If energy levels were continuous, the current would decrease smoothly. The sharp, periodic dips prove that atoms only accept energy in discrete packets, or 'quanta', as proposed by Bohr.
5. What is the difference between an elastic and an inelastic collision in the context of this experiment?
In this experiment:
- An elastic collision occurs when an electron collides with a mercury atom but doesn't have enough energy to excite it. The electron bounces off, retaining its kinetic energy.
- An inelastic collision occurs when the electron has at least 4.9 eV of energy. It transfers this exact amount of energy to the mercury atom, causing the atom to become excited. The electron loses its energy in this process.
6. Why does the electric current rise again after each dip?
After an inelastic collision causes a dip, the accelerating voltage continues to increase. This gives the electrons enough new energy to once again reach the collecting plate, causing the current to rise. The current increases until the electrons gain enough energy to cause another round of inelastic collisions, leading to the next dip at a higher voltage.
7. If neon gas were used instead of mercury vapour, would the results be the same?
No, the results would be different. Every element has its own unique set of characteristic energy levels. Neon's first excitation energy is around 18.7 eV, much higher than mercury's 4.9 eV. Therefore, the dips in the current would appear at different, higher voltages (multiples of 18.7V), demonstrating that quantized energy levels are a fundamental and unique property of each element.
