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))` ?

Explanation

Formula:
`hv=hv_(0)+(1)/(2)mv^(2)`
`hv=hv_(0)+(1)/(2)m(0)^(2)`
`hv=hv_(0)`
`lamda=4000Å`
`=4xx10^(3)xx10^(-19)=4xx10^(-7)m`.
V=0
`C=3xx10^(8)m//sec`.
`impliesv=v_(0)`
`:.v=(c)/(lamda)=(3xx10^(8))/(4xx10^(-7))=(3)/(4)xx10^(15)`
`=0.75xx10^(15)`
`=7.5xx10^(14)sec^(-1)`