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. Ultraviolet radiation approximately 4 eV to eV overlaps with the low end of the energy range of x rays, but UV is typically lower in energy. UV comes from the de-excitation of atoms that may be part of a hot solid or gas.
These atoms can be given energy that they later release as UV by numerous processes, including electric discharge, nuclear explosion, thermal agitation, and exposure to x rays. A UV photon has sufficient energy to ionize atoms and molecules, which makes its effects different from those of visible light. For example, it can cause skin cancer and is used as a sterilizer. But since UV does have the energy to alter molecules, it can do what visible light cannot.
One of the beneficial aspects of UV is that it triggers the production of vitamin D in the skin, whereas visible light has insufficient energy per photon to alter the molecules that trigger this production. Infantile jaundice is treated by exposing the baby to UV with eye protection , called phototherapy, the beneficial effects of which are thought to be related to its ability to help prevent the buildup of potentially toxic bilirubin in the blood. Short-wavelength UV is sometimes called vacuum UV, because it is strongly absorbed by air and must be studied in a vacuum.
Calculate the photon energy in eV for nm vacuum UV, and estimate the number of molecules it could ionize or break apart. According to Table 1, this photon energy might be able to ionize an atom or molecule, and it is about what is needed to break up a tightly bound molecule, since they are bound by approximately 10 eV.
This photon energy could destroy about a dozen weakly bound molecules. Because of its high photon energy, UV disrupts atoms and molecules it interacts with. One good consequence is that all but the longest-wavelength UV is strongly absorbed and is easily blocked by sunglasses. Damage to our ozone layer by the addition of such chemicals as CFCs has reduced this protection for us.
The range of photon energies for visible light from red to violet is 1. These energies are on the order of those between outer electron shells in atoms and molecules. This means that these photons can be absorbed by atoms and molecules. A single photon can actually stimulate the retina, for example, by altering a receptor molecule that then triggers a nerve impulse. Photons can be absorbed or emitted only by atoms and molecules that have precisely the correct quantized energy step to do so.
Figure 5. Why do the reds, yellows, and greens fade before the blues and violets when exposed to the Sun, as with this poster? The answer is related to photon energy. There are some noticeable differences in the characteristics of light between the two ends of the visible spectrum that are due to photon energies.
Red light has insufficient photon energy to expose most black-and-white film, and it is thus used to illuminate darkrooms where such film is developed. Since violet light has a higher photon energy, dyes that absorb violet tend to fade more quickly than those that do not. See Figure 5. Take a look at some faded color posters in a storefront some time, and you will notice that the blues and violets are the last to fade.
This is because other dyes, such as red and green dyes, absorb blue and violet photons, the higher energies of which break up their weakly bound molecules. Complex molecules such as those in dyes and DNA tend to be weakly bound. Blue and violet dyes reflect those colors and, therefore, do not absorb these more energetic photons, thus suffering less molecular damage.
Transparent materials, such as some glasses, do not absorb any visible light, because there is no energy step in the atoms or molecules that could absorb the light. Since individual photons interact with individual atoms, it is nearly impossible to have two photons absorbed simultaneously to reach a large energy step. Assuming that Power is energy per unit time, and so if we can find the energy per photon, we can determine the number of photons per second.
This will best be done in joules, since power is given in watts, which are joules per second. The power in visible light production is This incredible number of photons per second is verification that individual photons are insignificant in ordinary human experience.
It is also a verification of the correspondence principle—on the macroscopic scale, quantization becomes essentially continuous or classical. Instead, what Planck found by analyzing the spectra was that the energy of the hot body could only be lost in small discrete units. A quantum is the minimum quantity of energy that can either be lost or gained by an atom. An analogy is that a brick wall can only undergo a change in height by units of one or more bricks and not by any possible height.
Planck showed that the amount of radiant energy absorbed or emitted by an object is directly proportional to the frequency of the radiation. A small energy change results in the emission or absorption of low-frequency radiation, while a large energy change results in the emission or absorption of high-frequency radiation. What is the energy of a photon of green light with a frequency of 5. Step 1: List the known quantities and plan the problem. Step 3: Think about your result.
While the resulting energy may seem very small, this is for only one photon of light. Visible quantities of light consist of huge quantities of photons.
Recall that a hertz is equal to a reciprocal second, so the units agree in the equation. Use the link below to answer the following questions:.
Skip to main content. Electrons in Atoms. Search for:. Quantization of Energy Learning Objectives Define quantum. Describe the relationship between the amount of energy absorbed or released by an object and the frequency of the radiation observed.
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