Photon Energy and Power of BeamWatch
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.
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.
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?
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.
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.
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