Emax = hf

According to Einstein:

E_max = hf - W_o
eV or J	eV or J	eV or J

w_o = work function - work to pull away electron

(different for each metal)

hf = Energy of light used

E_max = 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!

E_max = hf - W_o
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. E_max (Maximum KE of photoelectron)

Frequency

Threshold frequency of:

Metal A?

f_o = 1.0 X 10¹⁴Hz

Metal B?

Frequency

f_o = 2.5 X 10¹⁴ Hz

Metal C?

Frequency

f_o = 3.75 x 10¹⁴ Hz

Which metal has the
greatest work function?

Frequency

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

* W_o = hf_o *

_{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 (W_o) of particular metal = 5.0 x 10^-19 J Find the threshold frequency (f_o)

( h = 6.63 x 10^-34Js )

W_o = hf_o

f_o = w/h

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

f_o = 7.5 x 10¹⁴ Hz

Since KE_max= hf - w_o

Then …

KE _max= hf - hf_o

(Use equation if given only frequencies)

PHOTON BY PHOTON DEVELOPMENT
OF A PHOTOGRAPH -

Optics Research Group