Nature of Light

Imath A.Latheef

Electromagnetic waves

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

other

Electromagnetic spectrum

• SpectrumThe ordered way a something arranged

(Ascending or descending)The EM waves which are arranged according to

wavelength of frequency is called EM Spectrum

EM Spectrum

Wave properties

• Reflection• Refraction• Diffraction• Interference • Polarization

Planks Law

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

Photon Photon

Plank stated that energy of a photonGiven 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 HzSolution E= hf E= 6.63x 10-34Jsx 2.5 x1015S-1

• 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 PhotonE=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)

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 3Find the energy of a photon with wave length of 400nm.Solution:-E=hf but C =fE= h x c/E=6.63 x 10-34 Js x 3.0 x108/(400 x 10-9) s-1

E= (………………………………..)

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.

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

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

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

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

• 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.E6.63 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

Stopping potential

the potential must be applied between a anode and the cathode to stop electron emissionDue to photo electric effect

The metal plate applied (+) potential to stop the photo electrons

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/hExample : 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

• 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 electron

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 KEOf the electron ( stopping potential Represented by eVs)

The gradient is equal to planks constant

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

Example –Jan 2010

Energy levels of an Atom

The cells of electrons levels is called as energy levels an atom,,

Excitation of electrons

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

de excitation of electrons

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

Atomic Spectra

• Continuous spectrum:- if the spectrum with all frequencies or wavelength with no gaps, then it is a continuous spectrum

400nm 700nmThe continuous spectrum of white light

An complete black body (fully black ) in higher temperature will emit continuous spectrum

Black body radiation

Black body

Continuous spectrum which has all wave length emission by black body

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

Missing frequencies /wave length

In abortion spectrum black lines in coloredBackground

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

Different excitation will make different frequencies disappear from Continuous spectrum

• 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

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

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

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

Summary

Particle nature of EM waves • Photo electric effect Threshold frequency,KE

of electrons ,Ect• Line spectrums (emission, absorption )

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