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

[XTEND]

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

 

[rock.freak667]

Your value for E is wrong.

h=6.626068 × 10^-34 m²kg/s

 

[XTEND]

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

 

[ideasrule]

okay.

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

You forgot to convert nm to m.

 

[rock.freak667]

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.

 

[XTEND]

1.9878E-25/4.2E-7 = 4.7328E-19

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

Thank You!