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Electrons are emitted with zero velocity from a metal surface when it is exposed to radiation of wavelength `4000Å` . What is the threshold frequency `(upsilon_(0))` ?

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Velocity of electron is 'Zero ' means KE = 0 ,
`thereforehupsilon=hupsilon_(0) " (or) "upsilon= upsilon_(0)`
`thereforeupsilon_(0)=(c)/(lamda)=(3xx10^(10))/(4000xx10^(-8))`
`" " [lamda4000Å=4000xx10^(-8)cm]`
`=(3)/(4) xx10^(15)`
`=0.75xx10^(15)=7.5xx10^(14)"Hz"`
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