Working out how many complete waves are sent out.

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AI Thread Summary
To determine how many complete waves are emitted by a light source lasting 0.04 μs with a wavelength of 600 nm and a speed of light at 3×10^8 m/s, the frequency is calculated using the formula f = v/wavelength. The correct frequency calculation reveals that the initial attempt led to an incorrect period of 2000 seconds, which is unrealistic for light waves. After correcting the calculations, the number of complete waves emitted is found to be 2.0×10^7. The discussion highlights the importance of careful calculation and verification in solving physics problems. Understanding the relationship between frequency, wavelength, and time is crucial for accurate results.
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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!
 
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onecupoftea said:

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?
 
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!
 
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