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1. Nature of Light
Imath A.Latheef

2. Electromagnetic waves
• Electromagnetic waves have two components
a) electric field
b ) magnetic field
Where both will oscillate perpendicular to each
other

3. Electromagnetic spectrum
• Spectrum
The ordered way a something arranged
(Ascending or descending)
The EM waves which are arranged according to
wavelength of frequency is called EM
Spectrum

4. EM Spectrum

5. Wave properties
• Reflection
• Refraction
• Diffraction
• Interference
• Polarization

6. Planks Law
• Max Plank Stated that Electromagnetic waves
can be represented as the Energy Packets
.Each energy Packet is called “Photon”
Photon
Photon

7. Plank stated that energy of a photon
Given by the Equation
• E=hf where h-Planks Constant (6.626 × 10-34 Js)
• F –Frequency (S-1 or Hz)
Example :
Find the energy of a photon with a frequency of
2.5x1015 Hz
Solution
E= hf
E= 6.63x 10-34Jsx 2.5 x1015S-1

8. • Example 2:
The energy of a wave is 20 J. if the frequency of the
wave is 3.0 x 1014 S-1 ,find the total number of
Photons present in the wave?
Solution:-
Energy of one Photon
E=hf
E= 6.63x 10-34Jsx 3.0x1014S-1
E= (……………….)
Total Energy = N x Energy of a photon (N- Number
of photons)
20= N x 6.63 x10-34Jsx3.0x1014S-1
N= 20/(6.63 x 10-34Js x 3.0 x 1014S-1)

9. Speed of EM In free Space
• Speed of EM = 3.0 x 10 8 ms-1
• The speed of EM wave in Air also considered as same as
free space
• Example 3
Find the energy of a photon with wave length of 400nm.
Solution:-
E=hf but C =f
E= h x c/
E=6.63 x 10-34 Js x 3.0 x108/(400 x 10-9) s-1
E= (………………………………..)

10. Electron Volts
• The electron volts (eV) is the equal to
1.6 x10-19J
• 2.3 ev = 2.3 x 1.6 x10-19J
• 6.7 J = 6.7J/1.6 x10-19 eV
• Note – Quantum physics calculation will be
deal with eV.

11. Photo Electric effect (PEE)
• When a EM radiation introduced in to a metal
surface it was found the electrons are emitted
from the metal surface this effect is called
PEE.(the emitted electron called photo
electrons )
EM radiation
Photo Electrons

12. Observations on PEE
• When the frequency of the incident radian increased
the speeds of the photo electrons were high – high
energy photons
• When the intensity of the radiation increased the
number of ejected electrons increased –more
photons more electrons
• It was found that the one photon is responsible for
one electron ejection

13. Definitions :-Work function Ø
• The energy required to overcome the
attraction between an electron and a metal is
called the work function of a metal .
(measured in eV)
Example – work function of iron is 2.3eV
Work function of Sodium 1.3 eV

14. Photo Electric Equation
• Albert Einstein showed that the ejected
electron and the photon related with the
equation
h f = Ø + ½ MeV2
Where –h-Planks constant
F – frequency,
Ø-work function
Me –Mass of electron
V-Speed of electron
The photo electric equation is a
form of conservation of energy
that is energy of a photon equals
to work function and the kinetic
energy of a electron

15. • Example ,
A metal with work function 2.3 eV introduced
with a radiation with wavelength 200nm . Find
the K.E. of an electron and the speed of the
electron (Mass of electron 9.31 x10-31 Kg)
hf = ø + K.E
6.63 x 10-34 x 3.0 x108 J = 2.3 x1.6 x10-19 +KE
200 x 10-9
KE= ½ MeV2 can fine the speed of electron

16. Stopping potential
the potential must be applied between a anode and the cathode to stop electron emission
Due to photo electric effect
The metal plate applied (+) potential to stop the photo electrons

17. Threshold Frequency
• The energy of a photon required just to
overcome the attraction between the metal and
the electron called threshold Frequency
• Therefore energy required is equal to work
function
hfo = Ø
fo = Ø/h
Example : find the threshold frequency of a metal
with work function 1.9eV
fo = 1.9 x 1.6 x 10 -19/(6.63 x10-34) S-1 0r Hz

18. • Therefore the photo electric equation can be
denoted as
• h f = hf0 + ½ me V2
Work function
When the frequency increased the kinetic
Energy of the electron will be increased therefore the
stopping potential should be increased to stop the electro
h f = hf0 + ½ me V2
Rearranging equation
1/2 me v2 = hf - hf0
eVs = hf - hf0
y = mx + c
The stopping potential is equal to KE
Of the electron ( stopping potential
Represented by eVs)
The gradient is equal to planks constant

19. The line for frequency Vs stopping potential If we do for the two types of metals
(the gradient is constant so parallel lines )

20. Example –Jan 2010

24. Energy levels of an Atom
The cells of electrons levels is called as energy levels an atom,,

25. Excitation of electrons
When the electrons from normal energy state jumps for higher energy state
Is called excitation of electrons . En external energy should be absorbed by the
electrons to excite from lower to higher energy level

26. de excitation of electrons
The electrons from higher energy levels ( which excited ) coming to
lower energy state ( emits radiation ) is called de excitation

27. Atomic Spectra
• Continuous spectrum:- if the spectrum with
all frequencies or wavelength with no gaps,
then it is a continuous spectrum
400nm 700nm
The continuous spectrum of white light
An complete black body (fully black ) in higher temperature will emit
continuous spectrum

28. Black body radiation
Black body
Continuous spectrum
which has all wave length
emission by black body

29. Absorption spectrum
• When the continuous spectrum passed
through a gas it was found that the emerging
radiation with missing frequencies or
wavelength .the spectrum obtained is called
absorption spectrum

30. Missing frequencies /wave length
In abortion spectrum
black lines in colored
Background
What happened to formation of absorption spectrum : the missing
wavelength energy photons are absorbed by the electron to make excitations
Therefore wavelength missing in the spectrum

31. Different excitation will make different frequencies disappear from
Continuous spectrum

32. • The excitation occurs if the energy of a photon
is equal to the difference in energy levels an
atom
If the excitation from n=1 to n=2 the energy of the photon
E2- E1 =hf
-3.39-(-13.6) eV = 6.63 x 10-34 x f
(-3.39+ 13.6) x1.6 x 10-19 / 6.63 x 10-34 = f
So frequency of the missing wave can be found if know the excitation stage
The excitation occurs only if difference equal to energy of the photon . Other photons
energies will not give a rise to excitation . This shows the energy levels of
electrons are quantized

33. • Absorption spectrum is a observation
to prove that the energy levels of an atom is
quantized

34. Emission spectrum
• The excited atom must come back normal
state when the electron de excited from
higher to lower energy and spectrum is
obtained this spectrum is called emission
spectrum
Colored lines in dark background is due to emission spectrum .
Not all the wavelength observed in an emission spectrum this also indicate
that atoms Energy levels are Quantized

35. The frequency of the emitted
radiation
E2- E1 = h x f
(E2-E1)/h = f
Applications of photo electric effect :
Solar cells , Photo sensors ,radiation detectors ect
Applications of Line spectrum : CFL bulbs ,Laser lights production ,chemistry applications
Such as NMR,IR ,UV, Visible spectrometers
Line spectrum : both abortion and emission Spectrums called Line Spectra

36. Summary

37. Particle nature of EM waves
• Photo electric effect Threshold frequency,KE
of electrons ,Ect
• Line spectrums (emission, absorption )

38. Wave particle Duality
• Louis de Broglie originated the idea that moving electrons may exhibit
both particle and wave nature. He proposed that, analogous to photons,
the wavelength of the electron is given by:
• λ = h p = h/ m v
• where h is Planck's constant
• p = mv electron momentum.
This concluded that the particles has a wavelenght which is given
as De Broglie Wave length