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OBJECTIVES
INTRODUCTION TO THE TOPIC PHOTO-ELECTRIC EFFECT
WHAT IS PHOTO ELECTRIC EFFECT?
THEORY
EXPERIMENTAL ARRANGEMENT
EFFECT OF INTENSITY OF LIGHT
EFFECT OF POTENTIAL
EFFECT OF INTENSITY OF LIGHT ON STOPPING POTENTIAL
EFFECT OF FREQUENCY
ANY QUERIES?
ASSIGNMENTS
REFERENSES
ACKNOWLEDGEMENT

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TO ENABLE THE STUDENT TO KNOW ABOUT

PHoTO-ELECTRIC EFFECT

TO HELP THE STUDENT TO ANSWER A QUESTION IN CBSE WITCH IS FREQUENTLY ASKED FOR 3 OR 5 MARKS
SHOWING A VIRTUAL EXPERIMENT ON PHOTO ELECTRIC EFFECT

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PHOTO –ELETRIC EFFECT IS THE TOPIC TAKEN FROM THE CHAPTER NO12, DUAL NATURE OF RADIATION AND MATTER, OF CLASSXII SYLLABUS

THE PHENOMENON OF PHOTO ELECTRIC EFFECT WAS DISCOVERDED BY HEINRICH HERTZ IN 1887 .IN HIS EXEPERMENT HE INVESTIGATED THE PRODUCTION OF ELECTRO MAGNETIC WAVES .

HERTZ OBSERVED THAT THE ZINC PLATE LOST ITS CHARGE

WHEN IT WAS ILLUMINATED BY ULTRAVIOLET LIGHT .

HERTZ CONCLUDED THAT NEGATIVELY CHARGED PARTICLES WERE EMITTED BY THE ZINC PLATE UNDER THE ACTION OFULTRA VIOLET LIGHT.

AFTER THE DISCOVERY OF ELECTRONS BY J.J. THOMSON THESE PARTICLES WERE TERMED AS PHOTO ELECTRONS

THE PHOTO-ELECTRIC EFFECT INVOLVES CONVERSION OF LIGHT ENERGY IN TO ELECTRICAL ENERGY .

IT WAS FOUND THAT CERTAIN METALS LIKE ZINC,CADMIUM,MAGNESIUM,ETC…
RESPOND ONLY TO U.V LIGHT.HOWEVER ,SOME ALKALI METALS SUCH AS LITHIUM,SODIUM,POTASSIUM,CAESIUM AND RUBIDIUM WERE SENSITIVE EVEN TO THE VISIBLE LIGHT

IN 1905 ,ALBERT EINSTEIN(1879-1955) PROPOSED A NEW THORY CALLED QUANTA OF ENERGY OF RADIATION TO EXPLAIN PHOTO ELECTRIC EFFECT .

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PHOTO-ELECTRIC EFFECT IS THE PHENOMENON OF EJECTION OF ELECTRONS FROM A METAL PLATE WHEN LIGHT OF A SUITABLE WAVE LENGTH FALLS ON .

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EXPERIMENTAL ARRANGEMENT

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WHEN U.V LIGHT IS INCIDENT ON A NEGATIVELY CHARGED ZINC PLATE IT LOOSES ITS CHARGE SLOWLY.THE LOSS IS DUE TO THE EJECTION OF ELECTRONS FROM THE SURFACE OF THF ZINC PLATE UNDER THE EFFECT OF LIGHT.THE ELECTRONS EMITTED ARE CALLED PHOTO-ELECTRONS.

THE ALKALI METALS LIKE SODIUM,POTASSI UM AND CEASIUM ARE SENSITIVE TO VISIBLE LIGHT.THE LIGHT OF SHORTER WAVE LENGTH HAS MORE ENERGY AND IS THUS MORE EFFECTIVE IN PRODUCING THE PHOTO-ELECTR0NS.

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INTENSITY OF LIGHT

PHOTO-

CURRENT

EFFECT OF INTENSITY OF LIGHT ON PHOTO-CURRENT

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Effect of intensity of light on photocurrent

Keeping the frequency of the incident radiation and the accelerating potential fixed ,the intensity of light is varied and the resulting photoelectric current is measured each time.

The photocurrent increases linearly with intensity of incident light

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EFFECT OF POTENTIAL ON PHOTO-ELECTRIC CURRENT

STOPING POTENTIAL

PHOTO-ELECTRIC CURRENT I 3

I 2

I 1

- V0

RETARDING POTENTIAL

ANODE POTENTIAL

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Effect of frequency of incident radiation on stopping potential

Photo-electric current

Saturation current

Anode potential V

- V01

-V02

-V03

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EFFECT OF POTENTIAL ON PHOTO-ELECTRIC CURRENT

For a particular frequency of incident radiation, the minimum negative retarding potential vo given to the anode A for which the photocurrent becomes zero is called the cut-off or stopping potential

K max=evo

For a given frequency of the incident radiation, the stopping potential is independent of its intensity.

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EFFECT OF FREQUENCY OF INCIDENT RADIATION ON STOPPING POTENTIAL

The stopping potential vo varies linearly with the frequency of incident radiation for a given photosensitive material

There exists a certain minimum cut-off frequency vo for which the stopping potential is zero.

The maximum kinetic energy of the photoelectrons varies linearly with the frequency of incident radiation, but is independent of its intensity.

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VARIATION OF STOPPING POTENTIAL V0 WITH FREQUENCY OF INCIDENT RADIATION FOR A GIVEN PHOTO-SENSITIVE MATERIAL.

Stopping

Potential v0

FRIQUENCY OF INCIDENT RADIATION V

METAL A

METAL B

V > v1

V > v2

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Variation of stopping potential with frequency of incident radiation

For a frequency v of incident radiation, lower than the cut-off frequency vo, no photoelectric emission is possible even if the intensity is large enough

This minimum cut-off frequency vo. Is called the threshold frequency vo.it is different for different metals.

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SUMMARY

FOR A GIVEN PHOTO-SENSITIVE MATERIAL AND FREQUENCY OF INCIDENT RADIATION, THE PHOTO-ELECTRIC CURRENT IS DIRECTLY PROPORTIONAL TO THE INTENSITY OF LIGHT.

FOR A GIVEN PHOTO-SENSITIVE MATERIAL AND FREQUENCY OF INCIDENT RADIATION, SATURATION CURRENT IS FOUND TO BE PROPORTIONAL TO THE INTENSITY OF RADIATION WHERE AS THE STOPPING POTENTIAL IS INDEPENDENT OF INTENSITY

FOR A GIVEN PHOTO-SENSITIVE MATERIAL,THERE EXISTS A CERTAIN MINIMUM CUT-OFF FREQUENCY,CALLED THRESHHOLD FREQUENCY.BELOW WHICH NO EMISSION OF PHOTO-ELECTRONS TAKE PLACE.

THE PHOTO-ELECTRIC EMISSION IS AN INSTANTANIOUS PROCESS WITH OUT ANY TIME LAG.

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ANY QUERIES ?

WHAT IS PHOTO ELECTRIC

EFFECT ?

IT IS THE PHENOMENON OF EMISSION

OF ELECTRONS BY METALS WHEN

ILLUMINATED BY LIGHT OF SUITABLE

FREQUENCY

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HOME TASK�

The work function of caesium metal is

2.14ev.when light of frequency 6x10¹⁴ Hz is incident on the metal surface, Photoemission of electrons occurs what is the

A) Maximum kinetic energy of the emitted electrons
B) stopping potential and.
C) Maximum speed of the emitted photoelectrons?

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