How Many Photons Per Second Does a Blue Laser Emit?

[fball558]

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?

Homework Equations

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

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!

 

[Redbelly98]

To solve this type of problem, it helps to think about units carefully (and properly!).

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?

Homework Equations

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

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.)

 

[nlightner]

Your calculations for converting watts to eV per second seem correct. To find the number of photons emitted per second, you can use the formula E = hf, where E is the energy in Joules, h is Planck's constant (6.6e-34 Joule seconds), and f is the frequency in Hertz. In this case, the frequency of the laser can be calculated by dividing the photon energy (3.1 eV) by Planck's constant (6.6e-34). This gives a frequency of approximately 4.7e14 Hz.

Using the formula E = hf, we can now calculate the energy of each photon emitted per second. This would be 6.6e-34 x 4.7e14 = 3.1e-19 Joules.

To find the number of photons per second, we can divide the power (0.8 watts) by the energy of each photon (3.1e-19 Joules). This gives us approximately 2.6e18 photons per second.

Therefore, this laser emits approximately 2.6e18 photons per second.