Parable of A Jailed
Elmo E. is jailed at the surface of
a particular metal. The bail needed
to him to get out happens to be $50.
Each of the photon brothers carries
exactly the amount shown. Only 1
photon can enter the jail at a time.
Which photon(s) is(are) needed to
post bail for Elmo?
Blue has exactly the right amount to
bail Elmo out. When he goes into the
jail, he gets Elmo out, but then Elmo has
no money to spend.
What happens when Purple goes in?
Purple gets Elmo out and gives him
$50 extra. Elmo gets out and has
some cash to celebrate (responsibly).
No matter what, Red, Orange, Yellow, and
Green can’t get Elmo out. What if Red
gets 50 of his friends (all exactly like
him) to help Elmo out?
Or what if Orange gets 10 of his identical
friends to help out?
REMEMBER - Only one photon can
enter the jail at a time, and they can’t
pool resources. Elmo is stuck unless
Blue or Purple come by.
Electron needs bail to get out (work
Only photons with energy equal to or
greater than the work function can get
the electron out
Extra energy appears as kinetic energy
of the electron
Energy of photon (E = hf)
Work function ()
Kinetic energy of electron (KE)
hf = + KE
Why so radical?
Classical physics said waves of any
frequency (color) could gradually give
energy to electron until it had enough
Classical physics said that an increase in
intensity of red light would give enough
energy to electron
Light in packets (“quanta” or “photons”)
Photons carry specific energy (hf)
Increasing intensity increases number
of photons of a particular f, but does
not increase energy per photon
Increasing number of red photons does
no good if 1 red doesn’t have enough E
Important points for setup
Voltage is the energy per charge
(Joules per coulomb)
Ammeter measures the current, or flow
Electron has a charge of 1.6 x 10-19 C
Energy can be measured in electronvolts (eV) [The energy needed to move
one electron through one volt.]
Get into groups to study the
Discuss and answer the questions.