# Homework Help: Power of a light source

1. Apr 4, 2014

### googlyeyes

1. The problem statement, all variables and given/known data

The WSFM radio station (which broadcasts at 101.7MHz) emits 1.78x10^29 photons per second and has an effective radiated power of 12kW.

If the same number of photons were emitted as visible light, what would be the power of the light source?

2. Relevant equations

P=W/t

3. The attempt at a solution

P = (1.78x10^29)x[(6.626x10^-34)x(3x10^8)/λ]
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 4, 2014

### BOYLANATOR

Strictly speaking, for dimensions the right hand side should be "per second".

Is the problem that you don't know the wavelength?

3. Apr 4, 2014

### googlyeyes

There are actually multiple parts to this question. Would you like me to type them up with my working for those as well (as that may also be wrong)

4. Apr 4, 2014

### BOYLANATOR

Well you can do if you need help with them, however, there is enough information here already for you to answer this question.

5. Apr 4, 2014

### googlyeyes

I will post it anyway:

(a) What is the value of the momentum of each radiated photon
(b) If an electron had the same momentum, how fast would the electron be travelling?
(c) How many photons does WSFM emit every second?
(d) If the same number of photons were emitted as visible light, what would be the power of this light source?

(a) p=2.246x10^-34
(b) p=mv thus v=(2.246x10^-34)/(9.10938x10^-31) thus v=0.000247m/s
(c) 1.78x10^29
(d) P = (1.78x10^29)x[(6.626x10^-34)x(3x10^8)/λ]

6. Apr 5, 2014

### BOYLANATOR

Ok, I'm not going to check the numbers but they look in the right ballpark. For (d) you are going to have to pick a suitable wavelength for visible light. This is common in these types of questions. Perhaps that helps?

7. Apr 5, 2014

### googlyeyes

Ok well im pretty sure that visible light has a wavelength between around 400-700nm but doesnt that mean there are so many wavelengths that could be chosen and thus many different answers to the question?

8. Apr 5, 2014

### BOYLANATOR

Yeah. You could work out the minimum and maximum and give the full range of answers, usually I just pick 500nm and run with that.