### 1B.9: Trouble with exponents and units

Posted:

**Sat Oct 13, 2018 2:33 pm**A lamp rated at 32 W emits violet light of wavelength 420 nm. How many photons of blue light can the lamp generate in 2.0 s? How many moles of photons are emitted in that time interval?

For this problem, I tried using v=C/λ to find the frequency, then multiply that by two seconds to get the number of photons, and the weird thing is that I got the number in the back of the book but the wrong exponent- 10^15 instead of 10^20. I also didn't use the given 32 W and the units don't seem to work out right. How do I do this problem to get the correct exponent and correct units? Also, as a side note, I seem to be getting wrong exponents a lot in problems about the photoelectric experiment, even when I get the right number. Any idea what I'm doing wrong?

For this problem, I tried using v=C/λ to find the frequency, then multiply that by two seconds to get the number of photons, and the weird thing is that I got the number in the back of the book but the wrong exponent- 10^15 instead of 10^20. I also didn't use the given 32 W and the units don't seem to work out right. How do I do this problem to get the correct exponent and correct units? Also, as a side note, I seem to be getting wrong exponents a lot in problems about the photoelectric experiment, even when I get the right number. Any idea what I'm doing wrong?