1.

The work functions `(W_(0))` of K, Na, Li, Mg and Cu are 2.25,2.30,2.42,3.70 and 4.80 eV respectively. How many of these metals do not undergo photoelectric effect when a radiation of wavelength 450 nm is allowed to fall on them? `(1eV=1.602xx10^(-19)J)`A. 2B. 1C. 3D. 5

Answer» Correct Answer - A
Ejection of photoelectron takes place when the absorbed quantum energy exceeds threshold energy.
`E_("absorbed")=(hc)/(lamda)`
`=(6.626xx10^(-34)xx3xx10^(8))/(450xx10^(-19))`
`=4.417xx10^(-19)J`
`(4.417xx10^(-19))/(1.602xx10^(-19))eV`
=2.75eV
Thus, Mg and Cu i.e., two metals will not eject photoelectron from the given radiation.


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