[luketapis]

I have to do this excercis:

A desk lamp produces 5.0J of energy per second (5.0W) in the form of blue light of wavelength 470nm. How many photons of blue (470nm) light are emitted in 1 second?


Are there any equasion for number of photons  emitted per second?

[JustinCh3m]

well, using E = hv  (v is actually "nu" = frequency), you can solve for frequency.  Frequency is in Hz which is a cycle/sec (or sec^-1).

Hopefully this puts you on the right track

[luketapis]

"well, using E = hv  (v is actually "nu" = frequency), you can solve for frequency.  Frequency is in Hz which is a cycle/sec (or sec^-1).
Ok so:

V=E/h
V= 5/6.626 x 10^-34 = 7.5460 x 10^33 s^-1

But it doesn't help me at all

[blaisem]

In your equation you used E=5 Joules.  Are you sure each photon emitted has an energy of 5 Joules?  A photon at a certain wavelength will only have one energy, and, as a hint, it is not 5 joules.  For a photon of light, a statement about its wavelength or frequency is a statement about its energy.  Wavelength, frequency and energy of a photon are all interrelated via E=hf.

[luketapis]

"Wavelength, frequency and energy of a photon are all interrelated via E=hf"
what is f hear?

[sjb]

"Wavelength, frequency and energy of a photon are all interrelated via E=hf"
what is f hear?

f is the frequency (the same as nu)...