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# Help on A2 laser/photons question Watch

1. Hi everyone,

Can anyone help with question 14.a)iii). I understand the photons per second is worked out by calculating useful power output/E of one proton. However, on the mark scheme it shows the power output as 3x10^-3. I don't understand how they have gone from 3mW to 3x10^-3 W... surely it would be 3x10^6 W? Although when I use this value, I get an energy efficiency of around 652000% which proves I am making the mistake, I just don't see where...

Thanks

Attachment 524701524713
Attached Images

2. (Original post by danielcb12)
Hi everyone,

Can anyone help with question 14.a)iii). I understand the photons per second is worked out by calculating useful power output/E of one proton. However, on the mark scheme it shows the power output as 3x10^-3. I don't understand how they have gone from 3mW to 3x10^-3 W... surely it would be 3x10^6 W? Although when I use this value, I get an energy efficiency of around 652000% which proves I am making the mistake, I just don't see where...

Thanks

Attachment 524701524713
http://astro.unl.edu/classaction/tab..._prefixes.html

prefix of m (lower case) means milli i.e. 10^-3
prefix of M (upper case) means mega i.e. 10^6
3. (Original post by Joinedup)
http://astro.unl.edu/classaction/tab..._prefixes.html

prefix of m (lower case) means milli i.e. 10^-3
prefix of M (upper case) means mega i.e. 10^6

Thank you! Seems so obvious now!

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