How many 420nm photons are emitted per second by an LED with a power of 10 mW?

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Homework Help Overview

The problem involves calculating the number of photons emitted per second by an LED with a specified power output and wavelength of light. The context is rooted in the principles of photon energy and power calculations in the field of optics.

Discussion Character

  • Mathematical reasoning, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants discuss the calculation of photon energy using the equation E=hc/λ and how to derive the number of emitted photons from the LED's power. There are attempts to correct initial calculations and clarify unit conversions, particularly from nanometers to meters.

Discussion Status

The discussion includes multiple attempts to arrive at the correct number of photons emitted per second, with some participants questioning the accuracy of previous calculations and values used. A final calculation appears to align with the expected answer, but there is no explicit consensus on the correctness of earlier steps.

Contextual Notes

Participants note the importance of unit conversion and the accuracy of constants used in calculations. There is an emphasis on verifying exponent values and ensuring proper application of formulas.

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Homework Statement



An LED emits a light beam with a power of 10 mW. The wavelength of the light is 420 nm. How many 420nm photons are emitted per sec.?

Homework Equations



E=hc/lamda is 2.95E-9

P/E= Photons emitted per sec.

(10*10E-3)(2.95E-9)=2.952E-11


The Attempt at a Solution



2.95E-11 is wrong.

2.1E16 photons per/sec. is the answer
 
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Your value for E is wrong.

h=6.626068 × 10-34 m2kg/s
 
Last edited:
okay.

(6.626x10E-34)(3.0x10E8)=1.9878E-25 / 420nm = 4.7328E-28

(10x10E-3) / (4.7328E-28) = 2.1129E25

This is still not 2.1E16
 
XTEND said:
okay.

(6.626x10E-34)(3.0x10E8)=1.9878E-25 / 420nm = 4.7328E-28

You forgot to convert nm to m.
 
XTEND said:
okay.

(6.626x10E-34)(3.0x10E8)=1.9878E-25 / 420nm = 4.7328E-28

(10x10E-3) / (4.7328E-28) = 2.1129E25

This is still not 2.1E16


Check over your exponents in the part in red, it is not E-28.
 
1.9878E-25/4.2E-7 = 4.7328E-19

(10x10E-3) / (4.7328E-19) = 2.1E16

Thank You!
 

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