Izhar Ullah (Reg#CSEE-XF12-200)

The Photo Electric Effects

OUTLINE:

Introduction. Laws of Photo Electric Effect.EINSTEIN’S EQUATION.APPLICATION.

INTRODUCTION:

 First observed by Heinrich Hertz in 1887,

the phenomenon is also known as the Hertz effect.

Hertz observed and then showed that electrodes illuminated with ultraviolet light create electric sparks more easily.

Definition:

The ejection of electrons from a metallic surface when the light of suitable frequency is allowed to fall on the surface.

The Photo Electric Effect

• This suitable frequency is called Threshold Frequency and the corresponding wavelength is called threshold wavelength.

• Work function: The work function is the energy required to remove an electron from the highest filled level in the Fermi distribution of a solid.

Laws of Photoelectric effect

• Photoelectric effect is directly proportional to intensity.• If the frequency of the incident light is less than the threshold frequency then no electron ejected, no matter what the intensity.

• The maximum kinetic energy of the electrons depend on the frequency of the incident light.

• The electrons were emitted immediately - no time lag.

EINSTEIN’S EQUATIONS FOR THEPHOTOELECTRIC EFFECT

Einstein's interpretation of the photoelectric effect results in equations which are valid for visible and ultraviolet light: energy of photon = energy needed to remove an electron + kinetic energy of the emitted electron

hν = W + EWhere

h is Planck's constant

ν is the frequency of the incident photon

W is the work function, which is the minimum energy required to remove an electron from the surface of a given metal: hν0

E is the maximum kinetic energy of ejected electrons: 1/2 mv2

ν0 is the threshold frequency for the photoelectric effect

m is the rest mass of the ejected electron

v is the speed of the ejected electron

EINSTEIN’S EQUATIONS FOR THEPHOTOELECTRIC EFFECT :

No electron will be emitted if the incident photon's energy is less than the work function.

Einstein’s equation

Km = E - W

Km:- maximum kinetic energy

E :- energy of photon’s

W :- work function of metal

WORK FUNCTIONS OF SELECTED METALS

Metal Ø (eV)

Na 2.46

Al 4.08

Cu 4.70

Zn 4.31

Ag 4.73

Pt 6.35

Pb 4.14

Fe 4.50

APPLICATION OF PHOTO ELECTRIC EFFECT:

Television Camera TubesLight-Activated CountersAutomatic DoorsIntrusion AlarmsTurn on Street Lights at Dawn