Quantum Mech.: Photons and Tanning Bed Safety

[kevnm67]

Homework Statement

Summer has arrived and you decide to go to the tanning bed. After arriving, you recall from physics class that the threshold for tissue damage from ultraviolet light corresponds to a wavelength of 300nm. You decide to inquire about the limits of exposure. You're informed that the photon energy produced by the tanning bed is between 3.8 eV and 3.9 eV.
A) What is the corresponding wavelength of the UV light?
a. 9.2 nm, 9.5 nm b. 325 nm, 315 nm c. 300 nm, 310 nm
B) Is this safe?
a. no b. yes c. not enough information
C) How many total photons is your skin exposed to if it receives just 5 mJ (milli-Joules) of total energy corresponding to photon at 3.9 eV?
a. 8 x 10^15 b. 4 x 10^15 c. 10 x 10^15

Homework Equations

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The Attempt at a Solution

a) choice B
b) choice B
c) not sure how to solve this one. Are they related by de Boglie?

 

[cupid.callin]

energy of 1 photon is 3.9 eV
convert it into joules

now total energy received = (no. of photons)(energy of each photon)

 

[kevnm67]

energy of 1 photon is 3.9 eV
convert it into joules

now total energy received = (no. of photons)(energy of each photon)

When I do that I get:
3.9eV x (1.6 x 10^-19 6.24 x 10^-19

Then multiplying that no. by 5x10^-3 doesn't give me the correct answer. What am I doing wrong?