Chapter # 20 Class: 12th
Atomic Spectra
(Short Questions with Answers)
Q1) Bohr’s theory of hydrogen atom is based upon several assumptions. Do any of these assumptions
contradict classical physics?
Answer: The Bohr assumptions about the hydrogen atom contradict with classical physics because Bohr used
Planck’s idea about radiation i.e., electrons revolve around the nucleus without radiating/absorbing energy.
Whereas classical physics states that electrons radiate energy continuously due to its circular motion around
the nucleus. Due to the emission of radiation energy, the electron would gradually come close to the nucleus
and ultimately should be absorbed in it, which is not possible.
Q2) What is meant by a line spectrum? Explain, how line spectrum can be used for the identification of
elements?
Answer: When electrons jumps from one orbit to another a photon comes out carrying energy equal to the
energy difference of the two states.
Different elements emit line spectrum of different set of wavelengths. This makes us easy to identify the
elements.
Q3) Can the electron in the ground state of hydrogen atom absorbs a photon of energy 13.6 eV and greater
than 13.6 eV?
Answer: When an electron absorbs a photon it gains the energy of the photon. Because an electron bound to
an atom can only have certain energies the electron can only absorb photons of certain energies. For example
an electron in the ground state has energy of -13.6 eV. The second energy level is -3.4 eV. Thus it would
take E2 − E1 = (-3.4 eV) – (-13.6 eV) = 10.2 eV to excite the electron from the ground state to the first excited
state.
If a photon has more energy than the binding energy of the electron then the photon will free the electron
from the atom – ionizing it. Thus the electron in the ground state of the hydrogen atom can absorbs a photon
of energy 13.6 eV and greater than 13.6 eV.
Q4) How can the spectrum of hydrogen contain so many lines when hydrogen contains only one electron?
Answer: A Hydrogen atom has only one electron but there are infinite number of energy states. The transition
of electron between every two of these states gives a new wavelength and, therefore, a new line in the
spectrum. As large number of different energy transitions are possible, therefore, a large number of different
Notes By: Ms. Maimoona Altaf Page 1
Chapter # 20 Class: 12th
lines can be emitted.
Q5) Is energy conserved when an atom emits a photon of light?
Answer: The law of conservation of energy holds when a photon of light emits from the excited atom, because
the energy absorbed by the atom during its excitation is exactly balanced by the energy emitted during its de-
excitation.
Q6) Explain why a glowing gas gives only certain wavelength of light and why that gas is capable of absorbing
the same wavelengths? Give a reason why it is transparent to other wavelengths?
Answer: The atoms of a glowing gas emit the radiations of wavelengths (or colors) which lie in its spectrum. It can
absorb the same wavelengths because only these wavelengths are suitable for excitation of its atoms.
The radiations of other wavelengths cannot excite atoms of this gas, so it is transparent to other wavelengths.
Q7) What do we mean when we say that the atom is excited?
Answer: If 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 an excited
state.
Q8) Can X-rays be reflected, refracted, diffracted and polarized just like any other waves? Explain.
Answer: X-rays are similar in nature to ordinary light with the difference of frequency and wavelength and so a
part of electromagnetic spectrum, so it can be reflected, refracted, diffracted and polarized.
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Chapter # 20 Class: 12th
Q9) What are the advantages of lasers over ordinary light?
Answer: Laser light is monochromatic i.e., consists of one wavelength, while ordinary light has a number of
wavelengths. The laser light is intense and coherent, so it does not spread while passing through a medium, its
energy can be focused at a point to get enough energy for welding, cutting and as surgical tool which ordinary
light cannot do.
Q10) Explain why laser action could not occur without population inversion between atomic levels?
Answer: The energy level can only be filled up to half the number of vacancies available. Population inversion
is the process in which electrons are lifted from ground state to a highly excited state by absorbing high energy
photons called "exciting photons". When the population inversion takes place, more than half of vacancies in
the metastable are filled.
Since laser action is a de-excitation process, for which excitation of electrons is required. A large number of
excited atoms in population inversion state emit a large number of coherent photons along the same direction
of motion. iI cannot take place without population inversion.
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