Lecture 2 - Elements of Modern physics_MODERN PHYSICS.pdf

Modern physics

1. MODERN PHYSICS004

2. LECTURE TWO
ELEMENTS OF MODERN PHYSICS

3. BLACKBODY RADIATION

4. **Classical versus Quantum theory of energy**

5. THE ULTRAVIOLET CATASTROPHE

11. Photoelectric Effect Equation
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??????
Where??????
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threshold
frequency

19. THERMIONIC EMISSION
Thermionic emission is when a metal will release electrons when it is heated.
This term can be broken down: “Therm-” means that this process relates to heating. “-ionic-” means that this
process relates to charge. “-emission” means that the process will emit something.

20. BOHR MODEL OF THE ATOM
In his Bohr model suggested that:
-Electrons could only travel in orbits at certain discrete distances from the nucleus with
fixed energies
-They can only gain or lose energy by moving from one energy level to another by
absorbing or emitting electromagnetic radiation with frequencyf. The difference in
energy level can be given by Planck’s relation:
∆??????=??????
2−??????
1=ℎ??????

21. THE LIMITATIONS OF BOHR’S ATOMIC MODEL
TheLimitationsofBohr’satomicmodelareasfollows:
Itcouldnotexplaintheatomicspectraofelementshavingmorethanoneelectron.
ItcouldnotexplaintheZeemaneffectwhenthespectrallinesaresplitintoclosely
spacedlinesundertheinfluenceofmagneticfield.
ItfailedtoexplaintheStarkeffectwhenthespectrallinesgetsplitintofinelinesunder
theinfluenceofanelectricfield.
AccordingtoBohr,thecircularorbitsofelectronsareplanar.Butmodernresearch
revealsthatanelectronmovesaroundthenucleusinthree-dimensionalspace.
ItdoesnotobeyHeisenberg’suncertaintyprinciple.Accordingtothisprinciple,itis
impossibletodeterminesimultaneouslytheexactpositionandmomentumofasmall
movingparticlelikeanelectronwithabsolutecertainty.However,accordingtoBohr,
electronsmoveinawell-definedcircularorbit.
Bohr’smodelcouldnotexplainshapesandgeometrymolecules.Itfailedtomake
correctpredictionsoflarge-sizedatomsandprovidedsufficientinformationonly
forsmalleratoms

22. ENERGY LEVELS OF THE ATOM -Definitions

23. ENERGY LEVELS
The electrons in an atom are trapped because they are attracted to the protons in the nucleus
Their energies are determined by which shell they are in: nearer the nucleus = less energy

27. INTERACTION OF RADIATION WITH MATTER

29. APPLICATION OF LASERS
The drilling of holes in the metals becomes easy with these lasers.
Lasers are used for bloodless surgery.
They find application in the military and are used in the target destination system.
Lasers are used to cut glass and quartz
Lasers are used in cancer diagnosis and therapy.
A laser helps in studying the Brownian motion of particles.
Lasers are used to store large amounts of information or data in CD-ROM.
Lasers are used in computer printers
Lasers are used in electronic industries for trimming the components of Integrated Circuits
(ICs).
Lasers are used in cosmetic treatments such as acne treatment, cellulite and hair removal.

30. FRAUNHOFERLINES
The Fraunhoferlines are a set of famous absorption lines named
after German physicist Joseph von Fraunhofer.
Whenlightfromthecentralcore(i.e.,photosphere)ofthesun
passesthroughthesun’satmosphere,certainwavelengthsare
absorbedbytheelementspresentsinthechromosphere(i.e.,the
outermostlayerofthesun),thiswillresultintheformationofdark
linesinthesolarspectrum,andthesesdarklinesthatarepresent
inthesolarspectrumareknownastheFraunhoferlinesofthesun
orjusttheFraunhoferlines.
The spectrum with the Fraunhoferlines of the sun also called the
Fraunhoferspectrum.

31. PRACTICE QUESTIONS
1.The particle nature of light is demonstrated by A. Photoelectric effect B. Speed of light C. Coloursof light D.
Diffraction of light
2. For an electron to be ejected from the surface of a metal, the radiation energy must be ------------------A.
greater than the work function of the metal B. less than the work function of the metal C. equal to the work
function of the metal D. greater than the rest mass energy of the electron
3. A metal having a work function of 5.76eV is illuminated with a radiation of 7.88eV. The kinetic energy of the
electrons emitted from the metal surface is A. 2.12eV B 1.37eV C. 45.39eV D. 13.64eV
4. An electron in the n = 5 energy level of hydrogen undergoes a transition to the n = 3 energy level. Determine
the wavelength of the photon the atom emits in this process. A. 1.28 x 10-6 m B. 2.37 x 10-6 m C. 4.22 x 10-7 m
D. 3.04 x 10-6 m
5. (I) The frequency of incident radiation. (II) The intensity of incident radiation. (III) The work function of the metal.
On which of the above parameters does the maximum energy of photoelectrons depend in photoelectricity?
A.(I) and (II) only. B. (I) and (III) only. C. (II) and (III) only. D. (I), (II) and (III).
6. The maximum kinetic energy of the photoelectrons emitted from a metal surface is 0.34eV. If the work function
of the metal is 1.83eV, find the stopping potential. A. 2.17V B. 1.49V C. 1.09V D. 0.34V 45.