According to Einstein:



Emax = hf - Wo

eV or J

eV or J

eV or J


wo = work function - work to pull away electron

(different for each metal)

hf = Energy of light used


Emax = Energy of photoelectron (Max KE given to electron)


Electronvolt - very small energy unit:
1 eV = 1.6 X 10-19 J





Einstein uses

 y = mx + b

to get his equation!




Emax = hf - Wo

eV or J


eV or J

eV or J





2. Threshold frequency - minimum frequency needed to expel electrons 



Explained by Einstein

Light Above
Threshold Frequency

Light Below
Threshold  Frequency

(Strong enough to emit electrons)

(Too weak to emit electrons)






Typical plot of Frequency vs. Emax (Maximum KE of photoelectron)






Threshold frequency of:

Metal A?





    fo = 1.0 X 1014 Hz






Metal B?







fo = 2.5 X 1014 Hz





Metal C?







    fo = 3.75 x 1014 Hz






Which metal has the
 greatest work function?






Work Function - energy needed to pull away electron from atom.



*  Wo = hfo  *


h = Planck's Constant

p.1 Reference






Ex 1) If a metal would only emit electrons when exposed to high frequency light what does that tell you about its work function?




Work function must be high






Ex) If the work function (Wo) of particular metal = 5.0 x 10-19 J Find the threshold frequency (fo)

( h = 6.63 x 10-34 Js )

Wo = hfo

fo = w/h

= (5 x 10-19 J/(6.63 x 10-34 Js)


fo = 7.5 x 1014 Hz 






Since KEmax= hf - wo



Then …


KE max= hf - hfo


(Use equation if given only frequencies)






Ref: P.G. Hewitt, Conceptual Physics, 9th ed.©2002 Pearson Education




Double Slit Experiment
One photon at a time

Optics Research Group

Photon - Particle



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