Such a light meter would be completely insensitive to red light, for example. See how light knocks electrons off a metal target, and recreate the experiment that spawned the field of quantum mechanics. Skip to main content. Introduction to Quantum Physics. Search for:. The Photoelectric Effect Learning Objectives By the end of this section, you will be able to: Describe a typical photoelectric-effect experiment.
Determine the maximum kinetic energy of photoelectrons ejected by photons of one energy or wavelength, when given the maximum kinetic energy of photoelectrons for a different photon energy or wavelength. Example 1. What is the energy in joules and electron volts of a photon of nm violet light?
Discussion The energy of this nm photon of violet light is a tiny fraction of a joule, and so it is no wonder that a single photon would be difficult for us to sense directly—humans are more attuned to energies on the order of joules. PhET Explorations: Photoelectric Effect See how light knocks electrons off a metal target, and recreate the experiment that spawned the field of quantum mechanics. Click to download the simulation. Run using Java. Conceptual Questions Is visible light the only type of EM radiation that can cause the photoelectric effect?
Which aspects of the photoelectric effect cannot be explained without photons? Which can be explained without photons?
Are the latter inconsistent with the existence of photons? Is the photoelectric effect a direct consequence of the wave character of EM radiation or of the particle character of EM radiation? Explain briefly. Insulators nonmetals have a higher BE than metals, and it is more difficult for photons to eject electrons from insulators. Discuss how this relates to the free charges in metals that make them good conductors.
If you pick up and shake a piece of metal that has electrons in it free to move as a current, no electrons fall out. Yet if you heat the metal, electrons can be boiled off. Explain both of these facts as they relate to the amount and distribution of energy involved with shaking the object as compared with heating it. Is this in the visible range? Find the longest-wavelength photon that can eject an electron from potassium, given that the binding energy is 2.
Is this visible EM radiation? What is the binding energy in eV of electrons in magnesium, if the longest-wavelength photon that can eject electrons is nm? Calculate the binding energy in eV of electrons in aluminum, if the longest-wavelength photon that can eject them is nm.
What is the maximum kinetic energy in eV of electrons ejected from sodium metal by nm EM radiation, given that the binding energy is 2. UV radiation having a wavelength of nm falls on gold metal, to which electrons are bound by 4. What is the maximum kinetic energy of the ejected photoelectrons? Violet light of wavelength nm ejects electrons with a maximum kinetic energy of 0.
What is the binding energy of electrons to sodium metal? UV radiation having a nm wavelength falls on uranium metal, ejecting 0. It was noted that these types of EM radiation have characteristics much different than visible light. We can now see that such properties arise because photon energy is larger at high frequencies. Figure 1. The EM spectrum, showing major categories as a function of photon energy in eV, as well as wavelength and frequency.
Certain characteristics of EM radiation are directly attributable to photon energy alone. Photons act as individual quanta and interact with individual electrons, atoms, molecules, and so on. The energy a photon carries is, thus, crucial to the effects it has. Table 1 lists representative submicroscopic energies in eV. When we compare photon energies from the EM spectrum in Figure 1 with energies in the table, we can see how effects vary with the type of EM radiation.
Figure 2. Gamma rays , a form of nuclear and cosmic EM radiation, can have the highest frequencies and, hence, the highest photon energies in the EM spectrum. This is sufficient energy to ionize thousands of atoms and molecules, since only 10 to eV are needed per ionization. When cell reproduction is disrupted, the result can be cancer, one of the known effects of exposure to ionizing radiation.
Since cancer cells are rapidly reproducing, they are exceptionally sensitive to the disruption produced by ionizing radiation. This means that ionizing radiation has positive uses in cancer treatment as well as risks in producing cancer. Since x rays have energies of keV and up, individual x-ray photons also can produce large amounts of ionization.
X rays are ideal for medical imaging, their most common use, and a fact that was recognized immediately upon their discovery in by the German physicist W. Roentgen — See Figure 2. Within one year of their discovery, x rays for a time called Roentgen rays were used for medical diagnostics. Roentgen received the Nobel Prize for the discovery of x rays. Once again, we find that conservation of energy allows us to consider the initial and final forms that energy takes, without having to make detailed calculations of the intermediate steps.
Example 1 is solved by considering only the initial and final forms of energy. Figure 3. X rays are produced when energetic electrons strike the copper anode of this cathode ray tube CRT. Electrons shown here as separate particles interact individually with the material they strike, sometimes producing photons of EM radiation.
Electrons ejected by thermal agitation from a hot filament in a vacuum tube are accelerated through a high voltage, gaining kinetic energy from the electrical potential energy.
When they strike the anode, the electrons convert their kinetic energy to a variety of forms, including thermal energy. But since an accelerated charge radiates EM waves, and since the electrons act individually, photons are also produced.
Some of these x-ray photons obtain the kinetic energy of the electron. The accelerated electrons originate at the cathode, so such a tube is called a cathode ray tube CRT , and various versions of them are found in older TV and computer screens as well as in x-ray machines. Find the maximum energy in eV of an x-ray photon produced by electrons accelerated through a potential difference of Electrons can give all of their kinetic energy to a single photon when they strike the anode of a CRT.
This is something like the photoelectric effect in reverse. The kinetic energy of the electron comes from electrical potential energy. We do not have to calculate each step from beginning to end if we know that all of the starting energy qV is converted to the final form hf. Gathering factors and converting energy to eV yields.
This example produces a result that can be applied to many similar situations. If you accelerate a single elementary charge, like that of an electron, through a potential given in volts, then its energy in eV has the same numerical value. Thus a Similarly, a kV potential in an x-ray tube can generate up to keV x-ray photons. Many x-ray tubes have adjustable voltages so that various energy x rays with differing energies, and therefore differing abilities to penetrate, can be generated.
Figure 4. X-ray spectrum obtained when energetic electrons strike a material. The smooth part of the spectrum is bremsstrahlung, while the peaks are characteristic of the anode material. Both are atomic processes that produce energetic photons known as x-ray photons. Figure 4 shows the spectrum of x rays obtained from an x-ray tube. There are two distinct features to the spectrum.
First, the smooth distribution results from electrons being decelerated in the anode material. A curve like this is obtained by detecting many photons, and it is apparent that the maximum energy is unlikely. This decelerating process produces radiation that is called bremsstrahlung German for braking radiation. The second feature is the existence of sharp peaks in the spectrum; these are called characteristic x rays , since they are characteristic of the anode material.
Characteristic x rays come from atomic excitations unique to a given type of anode material. They are akin to lines in atomic spectra, implying the energy levels of atoms are quantized. Phenomena such as discrete atomic spectra and characteristic x rays are explored further in Atomic Physics.
Connect and share knowledge within a single location that is structured and easy to search. Is there any difference between equation 1 and equation 2? And is equation 3 unrelated? I understand your confusion. They all are actually the same formulas. It is because lets start with the first equation,. Your first two equations are just the same.
That is the frequency of the photon that is coming in or being released. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more.
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