State the laws of photoelectrics effect. State Einstein.s photoelectric equations. The photoelectric threshold of a certain metal is `4000Å`. Determine the maximum energy of the ejected photoelectrons in electron volts by radiations of `2500Å`. (Given, `h=6.62xx10^(-34)Jsandc=3xx10^(8) ms^(-1)`).

Explanation

Given, `lamda_(0)=4000Å=4000xx10^(-10)` m
and `lamda=2500Å=2500xx10^(-10)` m
As `KE_("max")=hc(1/(lamda)-1/(lamda))`
`=6.62xx10^(-34)xx3xx10^(8)(1/(2500xx10^(-10))-1/(4000xx10^(-10)))`
`=2.98xx10^(-9) J=1.86 eV`