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F.Sc ICS Notes: Physics XII: Chapter 20 Atomic Spectra Exercise Short Questions:

FSc ICS Notes: Physics XII: Chapter 20 Atomic Spectra Exercise Short Questions:

Question 20.1 Bohr’s theory of hydrogen atom is based upon several assumptions. Do any of these assumptions contradict classical physics?
Answer 20.1 Bohr's assumptions about H-atom contradict with the classical physics. According to Bohr electron radiate no energy while revolving around the nucleus. Where as classical physics state that accelerated electron radiates energy continuously while revolving around the nucleus.

Question 20.2 What is meant by a line spectrum? Explain, how line spectrum can be used for the identification of elements?
Answer 20.2 When the atoms of a gas excited by supplying the energy from external source, they emit photons of specific wavelength on De-excitation. These photons of specific wavelength appear in the form of lines on a screen, which are well separated from each other, and is called line spectrum. Different elements emit photons of different wavelength. This mean the spectra of different elements are different. Hence we can identify the elements by their line spectrum.

Question 20.3 Can the electron in the ground state of hydrogen absorb a photon of energy 13.6 ev and greater than 13.6 eV?
Answer 20.3 The electron of H-atom in its ground state possesses energy equal to -13.6 eV. Hence H-atom will absorb photon of energy 13.6 eV and will get ionized. The extra energy of the photon will be taken up by the electron in the form of K.E.

Question 20.4 How can the spectrum of hydrogen contain so many lines when hydrogen contains one electron?
Answer 20.4 The hydrogen atom in the ground state has energy EO. But there are many other higher energy state above ground state. So when hydrogen atom is excited by supplying energy from external source it may jump into some higher energy state. When these electrons fall back then it may go to the ground state in different steps. Each transition will emit photon of particular wavelength. Hence we will get many lines in the line spectrum of hydrogen atom.

Question 20.5 Is energy conserved when an atom emits a photon of light?
Answer 20.5 Yes, The law of conservation of energy holds in the case when an atom emits a photon. Actually atom absorbs energy when they are excited to higher energy state. The same energy is re-emitted in the form of photon during the process of De-excitation.

Question 20.6 Explain why a glowing gas gives only certain wavelengths of light and why that gas is capable of absorbing the same wavelengths? Give a reason why it is transparent to other wavelengths?
Answer 20.6 Each and every element emits and absorbs radiation of some specific wavelength. Hence when certain radiations are incident on this element then it will absorb only those radiations, which it can also emit. Therefore, it is transparent to the radiations of other wavelength.

Question 20.7 What do you mean when we say that the atom is excited?
Answer 20.7  When an atom absorbs energy from external source, its electrons are lifted from ground state to higher energy state and as a whole the atom is said to be in the excited state.

Question 20.8 Can X-rays be reflected, refracted, diffracted and polarized just like any other waves? Explain.
Answer 20.8 Yes because X-rays are electromagnetic waves like an ordinary light but having frequency much higher than the ordinary light. Hence X-rays like ordinary light can show reflection, refraction, interference, diffraction and polarization properties.

Question 20.9 What are the advantages of lasers over ordinary light?
Answer 20.9 The laser light is intense, coherent and unidirectional so it does not spread out like ordinary light. Its energy can be focused at a point to get enough energy for welding, cutting and as surgical tool while ordinary light cannot do so.

Question 20.10 Explain why laser action could not occur without inversion between atomic levels?
Answer 20.10 If there is no population inversion between atomic levels then laser light will not obtained. A large number of excited atoms in population inversion state emit a large number of coherent photons along the same direction of motion. Which is not possible without population inversion.

Written By: Asad Hussain.