Working out how many complete waves are sent out.

[onecupoftea]

Homework Statement

A source of light emits a train of waves lasting 0.04 μs. The light has a wavelength of 600nm and the speed of light is 3×10⁸ms^-1. How many complete waves are sent out?
a)2.0×10⁷
b)4.5×10⁷
c)2.0×10¹⁰
d)4.5×10¹³

Homework Equations

f=v/wavelength (sorry, I couldn't find the wavelength symbol)

T=1/f

The Attempt at a Solution

Ok, this is what I tried:
f=v/wavelength
f=3×10⁸/600×10^-9
f=0.0005

T=1/f
T=1/0.0005
T=2000

then I did:
0.04×10^-6/2000

=2×10¹⁰

Which was so close to one of the answers; frustrating! Could somebody please explain to me what to do? It's driving me crazy. I'm finding it hard to carry on with the rest of the assignment because I keep going back to that question and staring at it, willing my brain to work!

 

[superdave]

The Attempt at a Solution

Ok, this is what I tried:
f=v/wavelength
f=3×10⁸/600×10^-9
f=0.0005

You entered this into the calculator wrong. Watch your parenthesis.

A big clue that something is wrong is that you have a period of 2000 s. If light has a really small wavelength and a fast speed, does it make sense that it should take over half an hour for one wave cycle?

 

[onecupoftea]

A-HA!
Thank you! I now get the answer 2×10⁷.
Yeah, I did look at the 2000s and think that was a tad wrong...
I'm now torn between being pleased that I knew how to work out the answer and the dismay of not being able to use a calculator.
Thank you again!