12th Class Physics Chapter 9 ATOMIC SPECTRA Short Question Answers

Bohr’s first postulate contradicts the classical physics. According to this postulate, an electron in an orbit revolving around the nucleus does not radiate energy by radiation. But, according to classical physics, an accelerated electron radiates energy due to its circular motion around the nucleus. As the revolving electron is accelerated, so it radiates energy, the electromagnetic waves. Due to the emission of radiations energy, the electron would gradually come close to the nucleus and ultimately should be absorbed in it.

When the atoms of an element are excited by absorbing the energy from incident photons, the excited atoms must return to their normal state by the emission of energy absorbed during excitation. The energy released forms a spectrum which consists of sharply defined spectral lines. Such a spectrum is called line spectrum. Each element gives its own characteristic lines of definite wavelength. Thus, an element can be easily identified by observing its spectrum.

The ground state energy of an electron in a hydrogen atom is 13.6 eV. A photon can be absorbed only if its energy is equal to the energy difference between the ground state and any one of the excited states, otherwise, it will remain unabsorbed. Therefore, the electron of the hydrogen atom in the ground state cannot absorb a photon of energy greater than 13.6 ev. It will absorb photon of energy which will lift the electron from its ground state to the state at infinity with E = 0.

When the energy is supplied to the atom of hydrogen, it will be excited then its single electron will jump from its ground state to some higher energy level. Now when it de – excites from higher level to ground level by several jumps, spectral lines of different wavelength are emitted. That is why, the spectrum of hydrogen contains many lines.

Yes, energy is conserved when an excited atom emits a photon .When an atom is excited, then energy is supplied to the electron by some external source. The same energy is emitted in the form of light photon when it returns back to its ground state. It means that the energy absorbed by the atom during excitation is exactly equally to the energy emitted during its de- excitation. Thus, law of energy is conserved i.e total energy remains the same.

Atom of gas have fixed energy level. When an electron jumps from a higher to a lower energy level an photon of fixed energy is emitted. Fixed energy means photon a particular wavelength. Therefore, a glowing gas emits a few particular wavelengths only depending upon the energy level. When white light passes through a gas, it absorbs only those photon which have energy equal to the difference of energy level in atoms of the gas. All other photons pass through the gas unabsorbed. In other words, gas is transparent for those photons.

If certain amount of energy is supplied of an atom by an external source, it will be raised up to one of the higher allowed state by absorption of energy. Then, the atop is said to be in an excited state. This state cannot exist for a long time, electron will return to its ground state after emitting certain radiations.

Yes, X – rays can be reflected, refracted diffracted and polarized as they are also electromagnetic waves of smaller wavelength and higher frequency. As they are similar in nature to ordinary light, therefore the X – rays posses all the properties specific to light waves. But their conditions may be different from that of ordinary light. For example, X – rays cannot be diffracted by diffraction grating but they are diffracted by crystals.

The lase light over ordinary light has the following advantages.
1. Laser light is monochromatic i.e consists of one wave length while the ordinary light has a number of wavelength.
2. Laser light is coherent i.e light waves are in same phase, while light waves has no phase coherence.
3. Laser light moves in the same direction, while ordinary light spreads in all directions.
4. Laser light is much more intense than ordinary light, while the ordinary light is less intense.
5. The laser light is produced due to stimulated emission of radiation, while ordinary light is produced due to spontaneous emission of light.

Any energy level can only be filled upto half the number of vacancies available. But when population inversion takes place, more than 50% vacancies in the met stable state become filled. This situation is therefore impossible; so that once population inversion is achieved all the electrons in the met stable state simultaneously jump to the ground level there by producing a plus of coherent photons.

When the atoms of an element are excited by absorbing the energy from incident photons, the excited atoms must return to their normal state by the emission of energy absorbed during excitation. The energy released forms a spectrum which consists of sharply defined spectral lines. Such a spectrum is called line spectrum. Each element gives its own characteristic lines of definite wavelength. Thus an element can be easily identified by observing its spectrum.

The photon is absorbed only of its energy is exactly equal to the excitation energy (10. 2 eV), otherwise not. So the photon cannot excite the atom. But the accelerated electron can lose its energy is full or in part. So the electron colliding with the same energy (10.5eV) can excite the atom when it gives a part of its energy to the atom.

When highly accelerated electrons collide with inner shells of the target, the electron in K – shell is knocked out. The vacancy thus created is filled by an L – shell electron and K – shell, an X – ray of even an electron from M – shell jupms into K –shell an X ray of even greater frequency is emitted and similarly from other shells fall into K – shell produce X –rays called K – series. In this way a spectrum of X – rays is absorbed.

The number of spectral lines (or the frequencies) of characteristic X – rays depends on the type of material used as target. If the target material has higher the atomic number, the greater will be the frequency of characteristic X -rays. In other words, we can say tha the frequency of low atomic number will be less than that of high atomic number element because the frequencies of charactreristic X –rays depends upon atomic number of the material used as target.

Laser.

The name laser stands for Light Almificationby Stimulated Emission of Radiation.
A laser is a device which produces very narrow intense beam of light having the following properties.
(i). It is monochromatic (of one wave – length)
(ii). It is coherent (crests and troughs of beam are in phase)

(iii). It is unidirectional (radiations of beam travel in the same directics)

Laser light.
1.Laser light is a monochromatic i.e consists of one wavelength.
2. Laser light is coherent i.e light waves are in same phase.
3. The laser light moves in the same direction
4. The laser light is produced due to stimulated emission of radiation.
5. Laser light is more intense than ordinary light.
Incandescent Light .
1. The ordinary light from incandescent body has a number of wave lengths.
2. Ordinary light has no phase coherent i.e waves are out of phase.
3. This light is emitted in all directions.
4. This light is produced due to spontaneous emission of light.
5. Its intensity is less.

Principle:- In a laser device, the basic principle used is to produce stimulated emission (radiation). In order to produce stimulation emission (radiation), a collection of atoms are excited (i.e electrons in them are raised to higher allowed state) by the absorption of energy supplied from some external source. The process in which de- excitation of an atom is caused by an incident photon with the emission of a second photon of the same energy and in phase with the incident photon is called stimulated emission.

(i). Laser are used for welding for detached retina in human eye. (ii). Laser are used to crush gallstone and kidney stone. (iii). A laser can be used to produce true three dimensional images called holograms. (iv). A laser can be used for the photographic recording of output data of a computer. (v). Laser also have military applications as laser guided missiles can destroy aircrafts and tanks. (vi). A high power laser can be used to induce nuclear fusion reaction. (vii). A laser has many uses in surgery. When the tumour is exposed to laser beam, the heat produced destroy the tissues quickly without any bleeding.

Laser light is monochromatic (i.e consists of one wavelength). It emits the photons of only one wavelength. Therefore, all the photons will have the same color due to the same wavelength. Hence, a laser emits only a particular color of light. Since each color has got its own wavelength.

There are different types of lasers whose power range varies from milliwatts to megawatts. Laser is classified into three major kinds according to their light magnifying substance (or radiations). 1. Solid lasers:- In solid lasers, a fluorescent crystal, such as that of a ruby glass or a semi – conductor is used as light amplifying substance. 2. Liquid Lasers:- In liquid lasers, a dye dissolved in methanol or a similar liquid is used. 3. Gas lasers:- In gas lasers, a gas or a mixture of gases is used as light magnifying substance (or medium). For example, helium – neon, argon ion and carbon dioxide gas lasers are most widely used as gas laders.

(a). Excitation potential:- The potential difference (voltage) applied to accelerate an electron from lower energy state to higher energy state for the required excitation energy is called excitation potential. The atom in an excited state does not stay there for a long time. On de – excitation the electron jumps down to some lower level, by emitting the energy difference of the two levels in the form of photons or spectral lines. (b). lionization potential:- The applied voltage (potential) due to which accelerated electron gets energy to ionize the atom is called ionization potential. For example, an electron in the ground state of a hydrogen atom process energy 13.6eV, thus the accelerating voltage required to ionization is it 13.6V. Hydrogen atom has only one value of ionization potential but a large number of excitation potentials. The atoms with several electrons can have more than of ionization potential. The ionization potentials of one element are equal to the number of electrons in its atoms. As helium has two electrons, it has two ionization potentials and so on.

Excitation energy:-  The amount of energy required to left an electron in an atom from its ground state to energy of the higher allowed states is called excitation energy. The atom is said to be in excited state. The excitation energy is equal to the difference of the energies of the excited state and ground state. The excitation energy to the atoms of an element can be supplied by heating the atoms or throwing light on the atoms.
(b). Ionozation Energy:- The amount of energy required to lift an electron in an atom from its ground state to the  state infinity(n = ) is called ionization energy.

. Postuates:-
1. The total energy of an electron in an allowed orbit remains constant as long as it remains in the same orbit. It means that an electron in an allowed orbit does not emit any energy.
(2). When an electron jumps from an orbit of higher energy E_n to an orbit of lower energy ‘E_p’ it will emit energy. The energy difference between the two orbits is emitted as a photon (or quanta) of energy hf, where f is the frequency of the photon. Thus, we have
hf = E_n – E_p
(3). An electron can revolve around the nucleus in only those orbits for which the angular momentum of the electron is integral multiple of h/2π, where ‘h’ is Plank’s constant. These orbits are called moving with velocity v_n in an orbit of radius r_n is given by mv_nr_n. Thus according to this postulate, we have
mv_nr_n  = n . h/2π

To excite an electron, we can supply energy is two ways. 1. By direct collision with the accelerated paricles 2. By shining light of the atom, i.e by photon of energy hf. Accelerated particles can transfer energy to the bound electrons in part or in full but the light exactly required frequency (f) must be shown for excitation; otherwise it will not be absorbed by the bound electrons. In this question the energy of the given photon is 10.5eV whereas the energy required for the excitation of the electron is 1-0.3 eV , therefore a photon cannot be used. Hence the electron, being is particle of energy 10.5 eV should be used.

The wavelengths for the Lyman series are given by the equation
1/λ  =  R_H (1/p²  – 1/n²)
but for Lyman series p = 1, therefore
1/λ  = R_H  (1/I²– 1/n²)   where n  = 2,3,4…………………………………..
Since at the room temperature the atoms of the hydrogen gas are in its normal state and their electrons are in the ground state, i.e. p = I, therefore the gas absorbs light of wavelengths equal to the lines in the Lyman series. This gas does not absorb the lines for the Balmer series, because for it p = 2.

The continuous spectra are obtained when transitions take place between positive energy states. Transition between positive energy state takes place in those atoms which are not free. In solids the atoms are not free but they are packed so tightly that their orbits overlap. On the other hand, the atoms are gases are isolated. Thus, the solids gives rise to continuous spectrum and the gases give rise to line spectrum on headind.

The jumps (or radiations) of electrons in the hydrogen or other light elements produce the emission of spectral lines in the infrared, visible or ultraviolet regions of electromagnetic spectrum due to small energy differences in the transition levels. In heary atoms the electrons are divided in concentric shells labeled as K, LM, N, O etc, The K – shell is closed to the nucleus, the L –shell next and so on. Thus, the transistions of high energy shell electrons in heavy atoms gives rise to the emission of high energy X- rey photons. There are known as characteristic X – rays.

When fast moving electrons strike a metal surface, photons of higher frequency are emitted called X –rays. They are produced when target metal is bombarded by electrons. The production of X – rays is a reverse process of photoelectric effect. In 1895, X –rays were discovered by Dr. Rontgeon of Germany.

The branch of physics which deals with the study of spectra formed by the emission or absorption of electromagnetic radiations by atoms of the elements is called spectroscopy. It also includes the spectra produced by atoms. Generally there are three types of spectra which are given below:- (i). Continuous spectra (ii). Band spectra (iii). Discrete or line spectra i.e atomic spectra.

It is basically consists of X – rays source with several hundred oppositely adjusted detector. Each detector measures absorption of X –rays along a thin line through the subject. The entire system is linked through a computer that is why it is named as computerized axel topography (CAT) and is widely used as a source of medical diagnostic.

The X –rays produces a thin fan – shaped beam which is detected on the opposite side of the subject by several hundred detectors arranged in a line. Each detector measures absorption of X- rays along a thin line through the subject. The entire apparatus is rotated around the subject in the plane of the beam within a few second. The changing reactions of the detector are recorded digitally. A computer process this information and reconstructs a picture of different densities over an entire cross- section of the subject. Density differences can be detected with CAT scans.

Mete stable state is an higher energy state in which electrons remain longer than usual. A met stable state in which an excited electron can stay for longer time as compared with ordinary excited state. Hence, electron can take comparatively longer time to de –excite (fall to lower state) from met stable state. Also the transition of electron from or to the met stable state is more different as compared to other excited state.

A laser is a device which produces very narrow beam of light having the following properties. (i). It is monochromatic (ii). It is coherent (iii). It is unidirectional The light emitted by an ordinary light is not only incoherent but also emitted in all directions. So laser light is different from the ordinary light.

When the incident photon is absorbed by an atom in the ground state with the energy E, the atom is raised with energy E₂, this process is called stimulated absorption or induced absorption. Once in the excited state, two things can happen to the atom.
(i). It may decay by spontaneous emission in which the atom emits a photon of energy.
hf = E₂ – E₁ in any arbitrary direction.
(ii). The other way in the excited state is to decay by stimulated emission.

It is defined as “a process of speed up atomic transition to lower levels”. In this case, the incident photon of energy hf = E₂ – E₁ induces an atom to decay by emitting a photon that travels in the directions of the incident photon. For each incident photon we will have two photons going in the same direction, thus an amplified and an unidirectional coherent beam is produced.