CBSE-XI-Physics
44: X-rays
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- #Section : i
- Qstn #1When a Coolidge tube is operated for some time it becomes hot. Where does the heat come from?Ans : A Coolidge tube apparatus consists of a filament and a target. The filament is heated to produce electrons that are accelerated by applying an electric field between the filament and the target. When these accelerated electrons enter the target, they collide with the target atoms. In the process, the electrons lose their kinetic energy. A part of this kinetic energy is utilised for emitting X-rays and the remaining energy is absorbed by the target. Inside the target, the kinetic energy of the electrons is converted into heat energy. This raises the temperature of the target and hence, it heats the Coolidge tube.
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- Qstn #2In a Coolidge tube, electrons strike the target and stop inside it. Does the target get more and more negatively charged as time passes?Ans : An electron emitted from the filament undergoes a number of collisions inside the material and loses its kinetic energy before coming to rest. This energy is utilised to give out photons or eject electrons from the atoms of the target. These electrons move to the battery connected to the circuit. Thus, the target does not get more and more negatively charged as time passes.
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- Qstn #3Can X-rays be used for photoelectric effect?Ans : Yes, X-rays can be use for photoelectric effect. Photoelectric effect is the emission of electrons from a metal surface when the frequency of radiation is greater than the threshold frequency of the metal. For photoelectric effect using X-rays, the energy of the incoming X-ray photon should be greater than the work-function of the metal used.
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- Qstn #4Can X-rays be polarised?Ans : Only transverse waves can be polarised. Since an X-ray is a transverse wave, it can be polarised.
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- Qstn #5X-ray and visible light travel at the same speed in vacuum. Do they travel at the same speed in glass?Ans : Speed of light in any material medium is inversely proportional to the refractive index of the medium. Since refractive index of glass for X-ray is less than that for visible light, an X-ray will travel at a faster speed than visible light in glass.
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- Qstn #6Characteristic X-rays may be used to identify the element from which they are being emitted. Can continuous X-rays be used for this purpose?Ans : Characteristic X-rays are emitted due to the transitions of electrons among different shells. The wavelength of the X-rays emitted in these transitions have definite value for a particular element. But continuous X-rays are emitted due to the conversion of kinetic energy of an electron into photon, which varies from collision to collision and is independent of material. Hence, continuous X-rays provide no information about the element from which they are being emitted.
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- Qstn #7Is it possible that in a Coolidge tube characteristic Lα X-rays are emitted but not Kα X-rays?Ans : Kα X-rays are emitted due to the transition of an electron from the L shell to the K shell and Lα X-rays due to the transition of an electron from the M shell to the L shell. If Kα X-rays are not emitted, then the L shell will not be vacant to take the electron from the M shell. Hence, Lα X-rays will not be emitted. Therefore, it is not possible that in a Coolidge tube, characteristic Lα X-rays are emitted but not Kα X-rays.
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- Qstn #8Can Lα X-ray of one material have shorter wavelength than Kα X-ray of another?Ans : An Lα X-ray is emitted when an electron jumps from the M to the L shell, and a Kα X-ray is emitted when an electron jumps from the L to the K shell. Less energy is involved when an electron jumps from the M to the L shell than when it jumps from the L to the K shell. Also, wavelength of a photon is inversely related to its energy. Therefore, an Lα X-ray has higher wavelength than a Kα X-ray for the same material.
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- Qstn #9Can a hydrogen atom emit characteristic X-rays?Ans : The difference of energy levels in a hydrogen atom is small. Hence, it is not able to emit characteristic X-rays.
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- Qstn #10Why is exposure to X-rays injurious to health but not exposure to visible light, when both are electromagnetic waves?Ans : X-rays have more penetrating power compared to visible light. As a result, they can penetrate the human body and can also damage the cells of the body. Prolonged exposure to X-rays can lead to cancer or genetic defects.
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- #Section : ii
- Qstn #1An X-ray beam can be deflected
(a) by an electric field
(b) by a magnetic field
(c) by an electric field as well as a magnetic field
(d) neither by an electric field nor a magnetic fielddigAnsr: dAns : (d) neither by an electric field nor a magnetic field
Since X-rays do not have any charged particles, they are not deflected by an electric field or a magnetic field.
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- Qstn #2Consider a photon of a continuous X-ray coming from a Coolidge tube. Its energy comes from
(a) the kinetic energy of the striking electron
(b) the kinetic energy of the free electrons of the target
(c) the kinetic energy of the ions of the target
(d) an atomic transition in the targetdigAnsr: aAns : (a) the kinetic energy of the striking electron
In an X-ray tube, electrons are emitted by the filament when it is heated. An electric field generated by a DC battery between the filament and the target makes the electrons hit the target atoms with a very high speed. As a result, the electrons lose their kinetic energy to eject photons, which leads to a continuous emission of X-rays.
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