How many photons are produced in a laser pulse of 0.203 J at 553 nm?

8.59

⋅

10

17

Explanation:

You can start by figuring out the energy of a single photon of wavelength  

505 nm

=

505

⋅

10

−

9

m

.

To do that, use the equation

E

=

h

⋅

c

λ

Here

h

is Planck's constant, equal to  

6.626

⋅

10

−

34

.

J s

c

is the speed of light in a vacuum, usually given as  

3

⋅

10

8

.

m s

−

1

λ

is the wavelength of the photon, expressed in meters

Plug in your value to find--notice that the wavelength of the photon must be expressed in meters in order for it to work here.

E

=

6.626

⋅

10

−

34

J

s

⋅

3

⋅

10

8

m

s

−

1

505

⋅

10

−

9

m

E

=

3.936

⋅

10

−

19

J

So, you know that one photon of this wavelength has an energy of  

3.936

⋅

10

−

19

J

and that your laser pulse produces a total of  

0.338 J

of energy, so all that you need to do now is to find how many photons are needed to get the energy output given to you.

0.338

J

⋅

1 photon

3.936

⋅

10

−

19

J

=

8.59

⋅

10

17

photons

−−−−−−−−−−−−−−−−−

The answer is rounded to three sig figs.

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