## Exam 1 - Q4A

$E=hv$

Johana Jeon 1A
Posts: 30
Joined: Sun Feb 12, 2017 3:00 am

### Exam 1 - Q4A

"In 1.0 s, a 60 W bulb emits 11 J of energy in the form of infrared radiation (heat) of wavelength 1850 nm. What is the energy per photon of light emitted? How many photons of infrared radiation does the lamp generate in 1.0 s?"

Can someone walk me through this problem please?
I think I got the first part, but I'm not sure how to get the # of photons
Thank you

Sarah_Wilen
Posts: 62
Joined: Fri Jun 23, 2017 11:39 am
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### Re: Exam 1 - Q4A

There are two parts to this question: a) energy per photon b) # photons generated

To find the energy of the photon, I used this equation: $E_{photon}=\frac{hc}{\lambda }$

As for the values...
-Used Planck's constant for h
-Used speed of light constant for c
-Converted the given wavelength of 1850 nm into meters using the unit conversion: 10^9 nm in 1 m.

When I plugged and chug, this was the first part of the answer:
$E_{photon}=\frac{(6.626\cdot 10^{-34} J\cdot s)(2.99\cdot 10^{8} m/s)}{1850\cdot 10^{-9} m}=1.07\cdot 10^{-19} J per photon$

Now that I know how many joules of energy was used per photon, I can see how many joules of energy was emitted (given as 11 J) and use my own "unit conversion" where I know...
-11 J of energy was emitted
-1.07 x 10^-19 J of energy exists per photon

I did a "unit analysis" seeing how many photons were in the 11 J:
$11 J\cdot \frac{1.0 photon}{1.07\cdot 10^{-19} J}=1.0\cdot 10^{20} photons$
That's a lot of photons!

Hope this helps!

Sent from Linux.