# Photon energy question

#### fball558

1. Homework Statement

A certain laser outputs pure blue light (photon energy 3.1 eV) with power 800 milliwatts (0.8 watts). How many photons per second does this laser emit?

2. Homework Equations

1 Ws = 6.24151e18
might need E = hv or E = (hc)/fequency where h is 6.6e-34

3. The Attempt at a Solution

not sure if i did it write but i took the .8 multiplied by the 6.24e18 (this is converting Ws to eV) then i x this number by the 3.1 eV to get 1.5479e19
this should be eV/s. then i am lost. not sure if that is even right so any help would be great!!

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#### Redbelly98

Staff Emeritus
Science Advisor
Homework Helper
To solve this type of problem, it helps to think about units carefully (and properly!).

1. Homework Statement

A certain laser outputs pure blue light (photon energy 3.1 eV) with power 800 milliwatts (0.8 watts). How many photons per second does this laser emit?

2. Homework Equations

1 Ws = 6.24151e18
might need E = hv or E = (hc)/fequency where h is 6.6e-34

3. The Attempt at a Solution

not sure if i did it write but i took the .8 multiplied by the 6.24e18 (this is converting Ws to eV)
Actually, it says 0.8 is in units of W, or equivalently J/s. If we include the units in the calculation, we can see what are the units of the result:
(0.8 J/s) x (6.24e18 eV/J) = _____?​

then i x this number by the 3.1 eV to get 1.5479e19
Here, it helps to think of it as 3.1 eV/photon (i.e, eV per photon, the energy that each photon has).

Like I did before, include the units in the multiplication and see what the result is.

this should be eV/s. then i am lost. not sure if that is even right so any help would be great!!
I hope my suggestions help you out. If you follow them, compare the units in the answer with what is being asked (highlighted in blue, near the beginning of my post.)

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