Introduction To Atomic Physics

Question 1

What does atomic physics deal with? What discovery played an important role in its development?

Answer 1

Deals with the study of sub atomic particles such as electrons protons and neutrons
Discovery of electron and measurement of its charge and mass

Question 2

What is electron emission? Why are metals preferred for electron emission?

Answer 2

The liberation of electrons from the surface of a metal is electron emission.
Metals are preferred for electron emission since they contain a large number of free electrons.

Question 3

What is work function? What is range of work function for pure metals? What are the factors on which it depends?

Answer 3

The minimum amount of external energy that must be supplied to a free electron in order that it may be just emitted out of the metal surface is known as work function of that metal
Varies from 2 eV to 6 eV
Depends on nature of metal its purity and surface conditions

Question 4

What are the types of electron emission depending on the method external energy is provided?

Answer 4

Thermionic emission - the process of electron emission from a heated metal surface
Photoelectric emission - the process of electron emission when light of suitable frequency is incident on it.
Field emission - the process of electron emission by application of a strong electric field at the surface of the metal
Secondary emission - the process of electron emission from a metal surface by the bombardment of high speed electrons or other particles.

Question 5

Name a device in which thermionic emission is used. Name 2 thermionic emitters. What factors that thermionic current density depend on?

Answer 5

Vacuum tubes
Tungsten metallic oxides of barium and strontium
Temperature and work function of emitter.

Question 6

What is strength of field required to cause field emission?
Why is field emission called cold cathode emission?
Why is it not commonly employed

Answer 6

A very strong electric field of the order of a few million volt per centimetre.
Since emission of electrons takes place even at room temperature.
Because of the large electric fields needed.

Question 7

Who introduced the term spectrum? What is a spectrometer ?

Answer 7

Newton.

The instrument used for viewing and analysing a spectrum is known as a spectrometer.

Question 8

What is spectrum ?what is visible spectrum?

Answer 8

Orderly arrangement of wavelengths.

Spectrum of light that can be seen. With wavelength range from 4000 to 8000 Å

Question 9

How are spectra broadly classified?

Answer 9

Emission spectra - the sample is an emitter .i.e. it is luminous
Absorption - the sample is an absorber situated in the path of light from a luminous source.

Question 10

What are the types of emission spectrum?

Answer 10

Continuous
Line
Band

Question 11

What is continuous emission spectrum? Name atoms sources. Why is it called hot body spectrum?

Answer 11

It is a spectrum which appears as an unbroken luminous band containing light of all wavelengths over a continuous range.
Glowing filament of electric bulb, arc lamp, candle flame, incandescent solids.
As the temperature of the source determines the characteristics of the spectrum.

Question 12

what is a line emission spectrum?

give an example

Answer 12

line emission spectrum consists of a number of bright coloured lines against a dark background.
sodium vapour lamp , mercury vapour lamp, helium, hydrogen discharge tube.

Question 13

why are line spectra called atomic spectra?

do two elements ever give rise to identical line spectra?why is it considered a finger print of the element?

Answer 13

line spectra are due to the excited state of the atoms of the element. hence they are also called atomic spectra
the number of lines and their relative positions are characteristic of the element present in the source. no two elements give rise to identical spectra. hence it is a finger print of the element.

Question 14

what is the wavelength of the 2 characteristic lines of sodium source
what is the prominent line in mercury vapour lamp spectrum?

Answer 14

D1 and D2 of wavelength 5890 and 5896 A

green line of 5461 A is the prominent line in mercury spectrum

Question 15

what is band emission spectrum ? give examples.

Answer 15

band emission spectrum is a spectrum which consists of a large number of bright coloured bands of graded intensity seperated from one another by dark spaces.
light from CO2 discharge tube
flame of gas burner - bands corresponding to cyanogen CN
nitrogen oxygen also emit band spectrums.

Question 16

what is band head?and band tail? (band emission spectrum)

Answer 16

band spectrum consists of a number of bright bands
one end of the band is very bright and sharp. called band head.
the end of the band which is dull and diffuse is called band tail.

Question 17

why are band spectra called molecular spectra?

Answer 17

band spectrum is given out by a molecule due to vibration and rotation.. transition between molecular quantum.
caused due to excitation of the molecules of the substance. hence molecular spectrum.

Question 18

what is continuous absorption spectrum? how is it formed give example.

Answer 18

when a wide range of wavelengths are removed by absorption from a continuous spectrum, the resulting spectrum is called continuous absorption spectrum.
it is obtained by passing light from a source emitting continuous spectrum through matter in the solid or liquid state and at a lower temperature.
white light from carbon arc lamp is passed through blue or red plate all colours except blue or red respectively are absorbed

Question 19

what is line absorption spectrum? give example.

Answer 19

the spectrum consisting of a number of dark lines against a bright coloured background , obtained when white light is passed through vapours or gases having free atoms are called line absorption spectrum.
sodium vapour lamp interposed between slit of spectrometer and carbon arc lamp. (2 dark lines in yellow region)

Question 20

what is band absorption spectrum . give examples.

Answer 20

a band absorption spectrum consists of a continuous emission spectrum with certain portions missing
intense white light passed through - potassium permanganate solution - dilute solution of blood - iodine vapour containing iodine molecules - blue cobalt glass.

Question 21

What are Fraunhoffer lines? Who discovered them?

Answer 21

The solar spectrum is a continuous spectrum crossed by a number of dark lines called Fraunhoffer lines. They were discovered by wallaston .

Question 22

Why are Fraunhoffer lines called so? What is the importance of F D C lines?

Answer 22

Fraunhoffer independently discovered and studied the lines. He labelled eight of the most prominent lines in the order A B C D E F G H.
F line is in blue region λ = 4861 Å
D line yellow region 5893 Å
C line red region 6562 Å
F D and C are taken as constants in many spectroscopic studies.

Question 23

What is the use of studying Fraunhoffer lines?

Answer 23

Help predict the composition of sun’s atmosphere . Elements present in earth’s atmosphere. Elements like He Na Fe H Ca are believed to be present
He was discovered by study of these lines

Question 24

What is flash spectrum? Explain.

Answer 24

During a total solar eclipse Fraunhoffer lines which were dark appear as bright emission lines. This spectrum is called flash spectrum.
This is because when the moon covers the sun we get light from the surface of the sun. It is used for the confirmation of total solar eclipse.

Question 25

Who theoretically predicted the existence and propagation of electromagnetic waves? ( who proposed electromagnetic theory)
Who experimentally confirmed their existence?

Answer 25

James clerk maxwell in 1865

Heinrich rudolf hertz

Question 26

What is an electromagnetic wave?

Answer 26

It is a fluctuating electric and magnetic field that travels through space.
It transports momentum and energy. They are transverse non mechanical waves produced by accelerated charges.

Question 27

What is electromagnetic spectrum ?

Answer 27

The orderly distribution of electromagnetic waves in accordance with/ in ascending order of their frequency or wavelengths is called electromagnetic spectrum.