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This is a discussion on CLASS 12 PHYSICS-- Chapter 9: ATOMIC SPECTRA within the 12th forums, part of the Classes category; CLASS 12 PHYSICS Chapter 9: ATOMIC SPECTRA Short answer question: Q1. Bohr Theory of H-atom is based on several ...

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    Chapter 9: ATOMIC SPECTRA
    Short answer question:

    Q1. Bohr Theory of H-atom is based on several assumptions. Do any of them violate classical physics?

    Ans. The Bohr first postulates violate the classical physics, according to which an electron in an orbit revolving around the nucleus does not radiate energy. But according to classical physics when the electron accelerates radiate energy. As the motion of electron around the nucleus is accelerate motion. Therefore this electron will radiate energy in form of electromagnetic waves.

    Q2. How can the line spectrum be used for identification of elements?

    Ans. Each element gives its own characteristic lines of definite wavelengths, due to which the spectrum of different element is different. If we observe the spectrum the element can be identified.

    Q3. How can the spectrum of hydrogen contain so many lines where the H-atom contains one electron?

    Ans. The hydrogen atom contains only one electron, but it can be excited to many excited states by supplying energy to it. During de-excited, this electron come back to ground de-excitation, this electron come back to ground state by several jumps, at the result of which spectral lines of different wavelengths are emitted.

    Q4. Is the energy conserved when an atom emits a photon?

    Ans. Yes, energy is conserved when an excited atom emits a photon. To excite the atom, an external energy is supplied to it due to which electron raises to higher energy level. But on de-excitation to ground state this external energy absorbs by atoms is emitted in the form of energy of photon so that the total energy remains constant.

    Q5. The electron in an H-atom requires 10.2eV for excitation to higher energy level. A photon and an electron, each of energy 10.5eV, are incident on the atom. Which of these can excite the atom. Explain your answer.

    Ans. The electron of energy 10.5eV will be able to excite the H-atom to the energy state 10.2eV. The electron on collision with H-atom gives energy 10.2ev to excite it where the remaining energy 0.3eV is retained by the electron as kinetic energy.
    The photon on collision with atoms either loses all its energy or does not lose energy and passes through the atom unabsorbed as the energy of given photon 10.5eV is greater than the required energy 10.2eV, so this photon will not be able to excite the hydrogen atom. For excitation of an atom, the photon must possess energy exactly equal to 10.2eV.

    Q6. What do we mean when we say that the atom is excited?

    Ans. If certain amount of energy be supplied to an atom, its electrons will be raised up to one of the higher allowed states by the absorption of this energy such atoms is said to be in an excited state, and the atom is called excited atom.

    Q7. Explain the production of continuous X-ray in an X-ray tube.

    Ans. In characteristics x-rays spectrum there has been observed continuous background radiation from the target. This radiation is the result of sudden slowing down of the fast moving electron with target. One deceleration, the energy of the fast moving electron appears in form of photons called continuous x-rays or bremsstrsblung.

    Q8. Explain why laser action could not occur without population inversion.

    Ans. The energy level can only be filled up to half the number of vacancies available. But when the population inversion takes place and more than half of vacancies in the metastable are filled, this will make impossible to achieve the population inversion and hence laser could not takes place.

    Q9. Describe the atomic process in the target of X-ray tube where X-ray line spectrum is produced.

    Ans. When a high-energy electron hit the target and the electron of the K-shell of atoms of material of target is knocked out, there produces a vacancy of electron in K-shell, which is to be filled, by the electron from L, M or N-shell. The transition of the electron from L, M or N-shell released energy in the form of x-ray photon so we get line spectrum of x-ray.

    Q10. Explain the difference between laser light and light from an electric bulb.

    i) Laser light is the result of stimulated emission where the light from an electric bulb is the result of spontaneous emission.
    ii) Laser light is coherent and all photons are in phase where the light from the electric bulb is incoherent and photons are not in phase.
    iii) Laser light move in one direction where light from electric bulb move in all direction.
    iv) The laser light is much more intense from the light emitted from electric bulb.
    v) Laser light is monochromatic light from the electric bulb is not monochromatic.
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