Answers to Review Questions: Ch # 18 Atomic Spectra

XII Karachi Board (BSEK)

Q.1 The bohr theory of hydrogen atom is based upon many assumptions. Do any of these assumption contradict classical physics?
Ans: The assumption in bohr’s theory that an electron moving around the nucleus in a certain orbit does not radiate energy is contrary to the classical physics.

Q.2 Why does the hydrogen gas produced in laboratory not glow and emit radiations?
Ans: A spectrum is given by the light emitted from an incandescent gas or vapors. For example electric discharge through a gas or hydrogen filled discharge tube

Q.3 Why are the energy levels of the hydrogen atom less than zero?
Ans:The energy levels of hydrogen are negative. This shows that an electron must do that amount of work to escape from electron or to over come the binding energy of that orbit.

Q.4 If hydrogen gas is bombarded by electrons of energy 13.6 eV, would you expected to observe all the lines of hydrogen spectrum?
Ans:the Hydrogen will get ionized because 13.6 is the energy of ground state and is equal to the ionization energy. As such no spectral lines will observed.

Q.5 Hydrogen gas at room temperature absorbs light of wave length equal to the lines in the Lyman series but not those of Ballmer series. Explain
Ans: Hydrogen gas at room temperature contains electron in the ground state so the energy it will absorbed must be equal to the difference of energy in the first orbit and  excited state .so the radiations are related to the Lyman (n=1)

Q.6 How are X-ray different from the visible radiations?
Ans: Because They
            * Are Penetrating        *Cause ionization in Gases     * Can eject photo electrons from metals                     * Produce fluorescence  * Can damage living tissues

Q.7 What property of X-ray makes them so useful in seeing otherwise invisible internal structures?
Ans:In solids atoms are arranged in a manner. which has distance in the order of X-rays. Hence crystal is used as transmission grating to produce diffraction of X rays. This crystallography helps us to locate the internal structure.

Q.8 Explain the difference between laser light and incandescent lamp(or bulb).


Laser light
Incandescent light
 Highly monochromatic
Mixture of several wavelengths
Coherent Waves
It is not coherent
It consists of parallel waves
It is emitted in all directions
Due to stimulated emission
Due to spontaneous emission








Q.9 Name some applications of laser.
Ans: 1-Three dimensional images of objects obtained by using laser in a process called Holography.2- As a surgical tool for welding detached retina.3- To perform precesion survey and length measurements 4-As potential energy source for inducing nuclear fusion reaction.5-For telephone communication along optical fibers.6-For cutting the metals

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