Dual Nature Of Radiation And Matter

- Learn important concepts of the chapter

Photoelectric effect

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When light of suitable frequency strikes upon a metal surface electrons are emitted from the surface. This phenomenon is called photoelectric effect.

Variation of photocurrent with potential for different intensity of incident light

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Light of certain wavelength and intensity ejects photo-electrons from a metal plate. Then the beam is replaced by another beam of smaller wavelength and smaller intensity. As a result
Emission of photoelectrons stops
No change occurs
KE of photoelectrons increases but the strength of photoelectric current decreases
KE of photoelectrons decreases but the strength of photocurrent increases

Variation of photocurrent with potential for different frequencies of incident light

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Einstein's photoelectric equation

Einstein's photoelectric equation :

The kinetic energy of the photoelectron coming out may be anything between zero and where is the energy supplied to the individual electrons.


Effect of potential on photocurrent

Photoelectric current is zero when the stopping potential is sufficient to repel even the most energetic photoelectrons with the maximum kinetic energy so that
For a given frequency of the incident radiation, the stopping potential is independent of its intensity.

Stopping potential

The smallest magnitude of the anode potential which just stops the photocurrent is called the stopping potential.
This potential should stop even the ost energitic photoelectron. Hence 
Tha value of maximum kinetic energy should be equal to
We know

Plot of stopping potential vs frequency of incident light

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We know the relation of stopping potential with frequency is 

where is the work function of the metal.
If we want to plot the stopping potential vs frequency , it will be a straight line with slope and negative y intercept

De-broglie wavelength

De Broglie's wavelength is the wavelength associated with a massive particle,hypothesized by De Broglie that explains Bohr's quantised orbits by bringing in the wave-particle duality.  It is written as
(de broglie wavelength)


Davisson and Germer experiment

Davison and Germer observed the diffraction of electrons by crystals. The setup consists of an electron gun firing electron beam onto a nickel target inside a vaccum chamber. The diffracted electrons are observed by the galvanometer.The experiment was performed by varying the voltage from 44V to 68V. It was observed that a strong peak appeared when the voltage was 54V at a scattering angle of due to constructive interference effects of the electrons. The wavelength of the matter waves was found to be 0.168 nm from diffraction measurements which matches that of de-Broglie wavelength at 54 V thus verifying the wave nature of electrons.