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The relationship between momentum and energy from Einstein’s theory of relativity is
E
2 
 
2 
E
0
2
, where, in this context, E
0
 mc
2
is the rest energy rather than the work function. a.
A photon is a massless particle. What is a photon’s momentum p in terms of its
energy E?
For photon m=0.
E
2 
( pc )
2  mc
2
, m = 0
E = pc p = E/c b. Einstein also claimed that the energy of a photon is related to its frequency by
E
 hf .
Use this and your result from part a to write an expression for the wavelength

of a photon in terms of its momentum p . c. pc
 hf p
 hf / c
 h /

Your result for part b is for a “particle-like wave.” Suppose you thought this expression should also apply to a “wave-like particle.” What is your expression for

if you replace p with the classical-mechanics expression for the momentum of a particle of mass m? Is this a familiar-looking expression? p
 mv
  h / mv
It is the deBroglie wavelength
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