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## Homework Statement

Calculate the momentum of a 140 eV photon.

## Homework Equations

p = E/c

## The Attempt at a Solution

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First, convert eV to Joules

140eV (1.60 x10

^{-19}J)

= 2.24 x10

^{-17}J

Now for momentum:

p = mv

p = (E/c

^{2})v

p = E/c

In my course, however, it says for photons E = hc/ λ

so p = E/c

is p = hc/λ/c

p = h/λ

My question is is it acceptable to use p = E/c even though this is with a photon, as the question only gives its eV value? or is there some way that I can't think of to determine the wavelength, λ, to then be able to use p = h/λ?

I have written my answer using p = E/c below.

p = E/c

p = 2.24 x10

^{-17}J / 3.00 x10

^{8}m/s

p = 7.47 x10

^{-26}kgm/s

The momentum of the photon is .47 x10

^{-26}kgm/s.

I would really appreciate any help on this problem.