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Electromagnetic Radiation and Quantum Phenomena

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Electromagnetic Radiation and Quantum Phenomena

Electromagnetic radiation carries energy, which it gives to particles such as electrons, thus establishing a relationship between radiation as an energy carrier and particles. We can think of this as a force moving a car: the photons are the force, while the car is the particle. The relationship between both is more easily observed when the photons excite the electrons and make them jump from their orbits or even throw them out of the atom.

The discovery of the photoelectric effect

The relationship between radiation as an energy carrier and particles was discovered experimentally by Heinrich Hertz and others following him, including J. J. Thomson, Philipp Lenard, and Robert Millikan.

A series of experiments using metallic plates and light to excite electrons were performed to observe the relationship between them and photons.

The theory behind the phenomenon was later explained by Albert Einstein and Max Planck, who finalised the concept of what is now known as the photoelectric effect.

The experiments of Heinrich Hertz

German physicist Heinrich Hertz performed some experiments using electrically charged surfaces with a gap between them. In these experiments, two metallic surfaces had different electric charges, thus causing a voltage difference. When the charge difference is large, an electric spark occurs, and electric charges flow through the gap.

If UV light shines onto the charged surfaces, electric sparks occur easily. The reason for this was unknown at the time, but the concept of electricity jumping more easily when UV light shone onto the metals was of interest to the scientists.

Electromagnetic Radiation & Quantum Phenomena. Photons. Electrons. Photoelectric. StudySmarter Figure 1. During Hertzs experiments, UV light shone onto a charged metallic object, causing the electrons to move out of the plate. Source: Manuel R. Camacho, StudySmarter.

J. J. Thomsons discovery

British physicist J. J. Thomson discovered that the effect behind what Heinrich Hertz had observed was linked to the light shining onto the plates, i.e., that the UV light pushed the electric charges from one metallic surface to the other. He noted that the electric charges responsible for producing the electric sparks had the same mass/charge ratio as the electrons and that the particles jumped from the surface with a larger electric charge to one with a smaller charge.

The experiments of Philipp Lenard

Hungarian-German physicist Philipp Lenard ran experiments with two plates separated by a gap. The first plate had a source of light shining onto it and a second plate placed over it.

An electron jumped from the first plate to the second one due to the increase in voltage difference. Lenard then changed the light intensity to see if this had any effect on the electrons jumping. It was expected that the light would help the electrons to jump more easily and thus transmit energy.

However, the experimental results were negative. There was no relationship between the energy of the charges jumping between the plates and the intensity of the light.

Electromagnetic Radiation & Quantum Phenomena. Photons. Electrons. Photoelectric intensity. StudySmarter Figure 2. Experiments established that increasing the intensity of the light does not change the energy of the expelled electrons. Source: Manuel R. Camacho, StudySmarter.

The experiments of Robert Millikan

Later on, American experimental physicist Robert Millikan tried to disprove the theory of light being a particle. Millikan theorised that if the experiment was done in a vacuum and with care, no electrons would be produced.

However, Millikan found his ideas were not true, and electrons were indeed ejected after radiation impacted the metal. His experiments established that releasing a charged particle required the light to have a minimum wavelength. His experiments also demonstrated a connection between wavelength and frequency. Since wavelength and frequency are related, Millikan found that light needed to have a minimum frequency to release electric charges from the metallic plates surface. This value has been called the cut-off frequency.

The slope of the plotted data was subsequently used to obtain the value of Plancks constant.

Electromagnetic Radiation & Quantum Phenomena. Photons. Electrons. Photoelectric frequency. StudySmarter Figure 3. It was discovered that the light frequency impacted the energy of the expelled electrons, with larger frequencies, such as the UV spectrum, resulting in a higher amount of energy. Source: Manuel R. Camacho, StudySmarter.

The relationship between electromagnetic energy and the released charges

The experiments run by Millikan and others demonstrated that variations in the lights brightness did not affect the number of particles released.

It was only when they changed the type of light shining onto the plates that the particles were affected. Light having short wavelengths (blue light) with higher frequencies released more and faster particles, which proved that the light energy was responsible for the electron emission, as the energy is related to the lights frequency.

It was Albert Einstein and Max Planck who, based on these experiments, made some further substantial contributions to our knowledge.

The contribution of Albert Einstein

Well-known German-born theoretical physicist Albert Einstein, having observed some experiments on particle emission, was able to fill out the theory with some new ideas. The main one was that it was the light colliding with the electrons that gave them energy. However, The light colliding with the electrons does itself need a certain amount of energy to release a charged particle.

According to Einstein, light is made of small particles, which he called the particle of light but which are now known as photons. These photons are what gives energy to the released particles. It was discovered that the energy of the photons is equal to the frequency of the light multiplied by a constant.

Einstein called the small particles of which light consists quanta. In physics, the term quantised means to divide a value into small pieces of fixed values.

The contribution of Max Planck

While Einstein provided the idea of light being composed of small particles, German physicist Max Planck proposed that electromagnetic radiation consists of small chunks of energy. These chunks were called quantised energy from the Latin quantus, meaning quantity.

Electromagnetic Radiation & Quantum Phenomena - Key takeaways

  • Experiments run by physicists Heinrich Hertz, J. J. Thomson, Philipp Lenard, and Robert Millikan showed that charged particle emission occurs more easily when light having high frequencies shines onto charged metallic plates with a voltage difference between them.
  • Experiments demonstrated that the brightness of the light does not affect the particle emission, whereas the type/frequency of light does.
  • Later theories, formulated by Einstein and Planck, contributed to our knowledge by explaining that energy released by electromagnetic radiation is quantised, which is to say that it is composed of fixed small packages of energy.
  • The amount of energy needed for radiation to release electrons from a material is fixed and depends on the photons frequency, which is why it is known as the cut-off frequency.

Frequently Asked Questions about Electromagnetic Radiation and Quantum Phenomena

The quantum theory of radiation says that electromagnetic radiation consists of small fixed amounts of energy. Every radiation value contains a multiple of this amount where n is an integer.

The quantum nature of electromagnetic radiation is demonstrated by the photons that produce electromagnetic radiation. The photons have discrete values of energy, which is to say that they are quantised. This, in turn, means that electromagnetic radiation is also quantised.

Not directly. Electromagnetic fields are caused by charged particles. Electric fields are created by the force that the charged particles exert, which is only felt by other electrically charged particles. When these particles move, they also produce a magnetic field, which only affects other magnetic fields or charged particles.


As the energy of the particles is quantised, the field values also have a quantised nature.

Final Electromagnetic Radiation and Quantum Phenomena Quiz

Question

If you have two charged plates with a small gap and a voltage difference between them, do you have a potential difference?

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Answer

Yes, you do.

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Question

If you increase the potential difference between two metallic plates divided by a small gap, what happens when the difference is large enough?

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Answer

Charges will start to jump from one plate to the other.

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Question

Experiments run by Heinrich Hertz showed that UV light had an effect on the particles jumping from one plate to another. What was the effect that Hertz discovered?

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Answer

Hertz found that particles jumped more easily, causing an electric spark.

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Question

Name some physicists other than Heinrich Hertz, whose experiments helped us to understand the nature of the interactions between light and particles.

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Answer

 Philipp Lenard, J. J. Thomson, and Robert Millikan.

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Question

What happened when the light intensity was changed?

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Answer

Particles did not jump as easily.

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Question

What was found to affect the velocity of the particles jumping from one plate to the other in the experiments run by Hertz, Lenard, and others?

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Answer

The type of light used.

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Question

What type of light was used to make charges jump more easily from one plate to another?

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Answer

UV light.

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Question

According to the observed effects, it was not the light’s brightness that affected the particles but another property of light. Which property was that?

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Answer

Frequency.

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Question

Whose research helped to fill out the theory behind the electromagnetic phenomena and radiation?

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Answer

The research done by Albert Einstein and Max Planck.

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What was Albert Einstein’s key idea?

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Answer

It was that light impacting particles behaved itself like particles. It gives the particles it affects a fixed amount of energy that depends on the light frequency and the Planck constant.

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How was the Planck constant calculated from the experiments run by Millikan?

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Answer

It was worked out by calculating the slope of the data.

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Question

What is the other name given to the light particle?

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Answer

Photon.

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What is the name of the effect produced by the release of charged particles by photons?

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Answer

The photoelectric effect.

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What does the term ‘quanta’ mean?

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Answer

Quantity.

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Question

Do higher frequencies of light increase the charged particles’ velocity after being expelled from an atom?

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Answer

Yes, they do.

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Question

Name the scientist who conducted the two-slits experiment.

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Answer

Thomas Young.

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What does the concept of wave-particle duality say?

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Answer

It says that light and atomic particles can behave as both particles and waves.

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What is ‘de Broglie’s wavelength’?

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Answer

It is the wavelength that is associated with particles.

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What are the experiments that helped to discover wave-particle duality for electrons called?

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Answer

The diffraction experiments.

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What was the corpuscular theory?

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Answer

It is the theory that says that light is made of small particles traveling through space.

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According to the corpuscular theory, light travels through which medium?

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Answer

Aether.

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Name two effects that the corpuscular theory could not explain?

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Why light refracted and why it travelled slower in water.

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Do photons behave like particles or like waves?

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Answer

Photons can behave like both particles and waves.

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What was the effect observed by Thomas Young that made him realise that light behaved like a wave?

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Answer

An interference pattern.

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Question

Name three scientists that conducted diffraction experiments.

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Answer

Clinton Davisson, Paget Thomson, and Lester Germer.

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When, during a diffraction experiment, electrons hit a crystal, did they simply scatter or did they present a wave diffraction pattern?

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Answer

They presented a wave diffraction pattern.

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Question

De Broglie’s wavelength is inversely proportional to which particle property?

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Answer

Its momentum.

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Question

If de Broglie’s wavelength is inversely proportional to its momentum, it will also be inversely proportional to the particle’s other two properties. Name those properties.

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Answer

Velocity and mass.

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Question

Name the three processes that can occur when an electron impacts an atom.

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Answer

Ionisation, excitation, and beta decay.

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What is ionisation?

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Answer

Ionisation occurs when an electron or a photon impacting an atom have enough energy to remove an electron from the atom. This process also happens when the atom gains an electron.

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What happens during the excitation process?

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Answer

The electron or photon injects energy into an electron in the atom, making it jump to a new orbit with higher energy.

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What happens during the beta plus decay?

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Answer

A proton converts into a neutron.

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Question

What is the ground state?

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Answer

It is the energy state of an electron that is not excited.

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How can an electron move to an upper, excited level?

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Answer

By gaining energy.

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From where does an electron gain the energy to move to a new orbit?

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From an electron or a photon impacting the atom.

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Does the electron stay at the excited level after gaining energy?

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No, the atom will look for a stable state, releasing the excess energy and moving the electron back to its original position.

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What happens when the electron moves back to its ground level?

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Answer

It releases excess energy in the form of a photon.

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Question

Without making any calculations, if an electron injects energy to a hydrogen atom, making the electron jump from E0 to E3, how much energy does the electron in the atom gain?

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Answer

It gains energy equal to the difference between both energy levels.

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Question

If a photon impacts a hydrogen electron on its ground state and moves it to n=2 or the first excited state, how much energy was gained? To answer this, you should consult the table in the explanation.

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Answer

It gains 13.6-3.4[eV] or 10.2[eV].

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Question

If a hydrogen electron moves from an excited state n=3 to the ground state n=1, how much energy is emitted in the photon? To answer this, you should consult the table in the explanation.

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Answer

12.1 [eV].

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Question

What is the photoelectric effect?

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Answer

It is the emission of electrons after high-frequency light impacts a metallic plate.

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Question

Who first observed the photoelectric effect?

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Answer

Heinrich Hertz.

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Question

Name two scientists who contributed to explaining the theory behind the photoelectric effect?

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Answer

Albert Einstein and Max Planck.

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Question

What is the work function?

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Answer

It is the minimum energy needed to release an electron by a high-frequency photon.

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Question

Which light did Hertz use in his experiments?

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Answer

UV light.

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Question

Does the energy of the light depend on the brightness?

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Answer

No, it doesn’t.

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Question

What property of the light was found to eject the electrons more easily?

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Answer

The frequency.

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Question

Does the energy of the light depend on its frequency?

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Answer

Yes, it does.

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Question

The energy of a photon is equal to the product of a constant and a variable. Can you name them?

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Answer

The Planck constant and the photon frequency.

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Question

The energy of the photon releasing the electron is split in two. One part, the work function, removes the electron from the metal? What happens to the rest of the energy?

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Answer

It becomes the kinetic energy of the emitted electron.

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