# Nice easy question about Photons.

1. Jun 15, 2011

### Brammo

Estimate how many photons of visible light (500 nm) are emitted by a 100 W Bulb.

I get 2.5$\times$10$^{20}$ Photons/Second

Am I correct?

Thank you.

Last edited: Jun 15, 2011
2. Jun 15, 2011

### phyzguy

Two questions:

(1) Do you mean 2.5*10^20 photons/second?

(2) Does a 100W bulb emit 100W of light energy, or does it use 100W of electrical energy (or both)?

3. Jun 15, 2011

### Brammo

(1) yes, sorry. (2) I assume it means 100w of light energy. But what would the answer be if it was electrical?

4. Jun 15, 2011

### phyzguy

It doesn't mean 100W of light energy. An incandescent bulb is only about 4% efficient, and so only puts out about 4W of light energy.

5. Jun 15, 2011

### Brammo

Ok well these are practice questions written by a physics professor. The answer he has written at the bottom of the questions is approximately 10^20 photons/second. I just wanted to know where i had gone wrong if I had. The question is as above and its not something you need to pick apart I was just asking for a little bit of help is all. It does mean 100W of light energy. So am I correct or am I incorrect? The idea is for us to learn and understand the formula.

Substituting in the value of 4W give you 10^18 photons/second which is much further out.

6. Jun 15, 2011