Photons emitted per sec.

  • Thread starter XTEND
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  • #1
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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
 

Answers and Replies

  • #2
rock.freak667
Homework Helper
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Your value for E is wrong.

h=6.626068 × 10-34 m2kg/s
 
Last edited:
  • #3
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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
 
  • #4
ideasrule
Homework Helper
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okay.

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

You forgot to convert nm to m.
 
  • #5
rock.freak667
Homework Helper
6,230
31
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.
 
  • #6
19
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1.9878E-25/4.2E-7 = 4.7328E-19

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

Thank You!
 

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