How Many Photons Does a 100 W Bulb Emit Per Second?

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Discussion Overview

The discussion revolves around estimating the number of photons emitted by a 100 W bulb, specifically focusing on visible light at a wavelength of 500 nm. Participants explore the implications of the bulb's efficiency and the distinction between electrical and light energy output.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant estimates that a 100 W bulb emits approximately 2.5×1020 photons per second.
  • Another participant questions whether the 2.5×1020 photons per second refers to the correct notation and whether the 100 W refers to light energy or electrical energy.
  • A participant clarifies that incandescent bulbs are about 4% efficient, suggesting that only about 4 W of light energy is actually emitted.
  • There is a discussion about the implications of using 4 W instead of 100 W for calculating photon emission, leading to a different estimate of approximately 1018 photons per second.
  • One participant expresses confusion regarding the professor's practice questions and seeks clarification on their earlier estimate.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct number of photons emitted, as there are differing views on the efficiency of the bulb and the interpretation of the energy output. The discussion remains unresolved regarding the correct calculation based on different assumptions.

Contextual Notes

There are limitations regarding the assumptions made about the efficiency of the bulb and the definitions of light energy versus electrical energy, which affect the calculations presented.

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

I get 2.5\times10^{20} Photons/Second

Am I correct?

Thank you.

EDIT: Added units.
 
Last edited:
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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)?
 
(1) yes, sorry. (2) I assume it means 100w of light energy. But what would the answer be if it was electrical?
 
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.
 
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.
 
Brammo said:
Ok well these are practice questions written by a physics professor.

Then please post this in the appropriate section.
 

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