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Aillias
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#1
Report Thread starter 9 years ago
#1
Sorry for another question guys, but i'm working through my physics text book and i've encountered a problem.

A laser emits light of wavelength 635nm in a beam of power 1.5mW. Calculate the number of photons emitted by the laser each second.

Equations used:

C=f x lamda

Photon energy E=hf

Power of the beam = nhf

What I did, f = c/ lamda

f= (3 * 10^8) / (635 * 10 ^-9) = 4.72 x 10 ^ 14 Hz.

6.63 x 10 ^-34 x 4.72 x 10 ^14 = 3.1 x 10 ^ -19 J

I know for a fact that so far, I've got it all right, because it says so in the back of the book.

However, then,

P = nhf

n = p/hf

1.5 mW / (3.1 x 10 ^-19J) = 4.84 x 10^ 24

But the correct answer in the back of the book is 4.8 x 10 ^ 15

I've searched on the internet, and someone said you'd have to convert the 1.5 mW. so it becomes 1.5 ^ -3 which you then sub back in. I've checked and yes that's the correct answer.

So my question is WHY use 1.5 ^ -3. What has the 1.5 mW been converted to? What units? And why can't i just use 1.5mW as it is?
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Aillias
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#2
Report Thread starter 9 years ago
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It's alright.

I've realised that 1.5mW is actually milliwatts.

I'd thought that it was megawatts, which would have been MW. Ah. Now it makes a lot more sense.
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moonink
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#3
Report 4 weeks ago
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9 years later and here I am, looking at a copy of what I thought was going on - mW was actually milliwatts (not megawatts), (╯▅╰), I didn't expect that whatsoever to be the error in my calculations...

Thanks for posting your solution up ☺, I find most just solve the problem and don't post the solution - especially when there wasn't a large discussion following up on it.
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Joinedup
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#4
Report 4 weeks ago
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yeah for the prefix letters little m means milli and capital M means mega

generally little letter means little size... except kilo is little k... but thankfully there isn't a big K prefix to mix it up with
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