EXERCISE SHORT QUESTIONS
CHAPTER # 20: ATOMIC SPECTRA
Q # 1. Bohr's theory of hydrogen atom is based upon several assumptions.
Do any of these assumptions contradict classical physics?
Ans. Bohr's first postulate disagrees with the classical physics. According to this postulate:
An electron in an orbit revolving around the nucleus doesn't radiate energy by radiation. But according to the classical physics, an accelerated electron radiates energy due to its circular motion around nucleus.
Q # 2. What is meant by line spectrum? Explain, how line spectrum can be used for identification of elements?
Ans. A spectrum which consists of isolated sharp parallel lines, in which each line corresponds to a definite frequency and wavelength, is called line spectra.
Each element gives its own characteristic lines of definite wavelengths. Thus an element can be easily identified by observing its spectrum.
Q # 3. Can the electron in the ground state hydrogen absorb a photon of energy 13.6 eV and greater than 13.6 eV.
Ans. Yes it can absorb a photon of energy 13.6 eV and greater than 13.6 eV.
Since the ionization energy of the electron in the ground state of hydrogen atom is 13.6 eV. So by absorbing a photon having energy greater than 13.6 eV, ionization of H-atom will take place and the surplus energy of photon is taken away by electron as kinetic energy.
Q # 4. How can the hydrogen emission spectrum contain so many lines although it contains one electron only?
Ans. When H-atom de-excites, the electron will come from higher energy level to ground level by several jumps. As the result, photons of different wavelengths are emitted. That's why the spectrum of hydrogen contains so many lines.
Q # 5. Is energy conserved when an atom emit a photon of light?
Ans. The energy emitted during de-excitation is exactly equal to the energy absorbed by the atom during excitations. So the energy is conserved in this process, i.e., total energy remains the same.
Q # 6. Explain why a glowing gas gives only certain wavelength of light and why that is capable of absorbing the same wavelength? Give a reason why it is transparent to other wavelengths?
Ans. Atoms have fixed energy levels. When electron jumps from higher to lower energy level during de-excitation, photons of particular wavelengths are emitted.
On the other hand, when white light is passed through gas, it absorbs only those photons which have the energy equal to the difference of energy levels in atoms of the gas. All other photons pass through the gas un-absorbed. In other words, gas is transparent for those photons.
Q # 7. Why do you mean when we say that the atom is excited?
Ans. If the certain amount of energy is supplied to the electrons of an atom by an external source, it will be raised up to one of the higher allowed states by absorption of energy. Then the atom is said to be in excited state.
Q # 8. Can X-rays be reflected, refracted, diffracted and polarized just like any other waves? Explain.
Ans. Yes, X-rays can be reflected, refracted, diffracted and polarized as they are also electromagnetic waves of higher frequency and smaller wavelength. Therefore, the X-rays posses the entire properties specific to light waves.
Q # 9. What are the advantages of laser over ordinary light?
Ans. The laser light over ordinary light has following advantages:
- Laser light is monochromatic, while ordinary light has number of wavelengths.
- Laser light is coherent, while ordinary light has no phase coherence.
- Laser light moves in the same direction, while ordinary light spreads in all direction.
- Laser light is much more intense than ordinary light.
Q # 10. Explain why laser action could not occur without pollution inversion between atomic levels.}
Ans. In population inversion, more than 50% vacancies in the meta-stable states become filled. Then all the electrons in the meta-stable state simultaneously jump to the ground level, thereby producing a pulse of coherent photons. Without population inversion, laser action could not occur.
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