# Working out how many complete waves are sent out.

1. Nov 14, 2012

### onecupoftea

1. The problem statement, all variables and given/known data
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

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

T=1/f

3. 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!
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 14, 2012

### superdave

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?

3. Nov 14, 2012

### 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!