How long does it take a 1ms pulse of light to travel 20m

  • #1

Homework Statement


If you create a 1 ms light pulse by turning a flashlight on and off, how long will the pulse take to reach the other side of the room 20m away? (in air at standard T and P) what is the length, in m, of the pulse?

I realise that light has a wavelength and when you pulse it that wavelength shouldn't change. Really I just don't know the right equations to solve the problem

I'm struggling with this pre-lab homework as I'm taking a physics paper out of interest and cannot find any concise or simple enough explanations of the answer in the readings I've been given or on the internet. Any help would be greatly appreciated.

Cheers
 

Answers and Replies

  • #2
Bystander
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1 ms light pulse by turning a flashlight on and off, how long will the pulse take to reach the other side of the room 20m away? (in air at standard T and P) what is the length, in m, of the pulse?
Do you know what light speed is? Do you know what "1 ms" is? Do you know what a "rate equation" is?
 
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  • #3
berkeman
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Welcome to the PF.

Homework Statement


If you create a 1 ms light pulse by turning a flashlight on and off, how long will the pulse take to reach the other side of the room 20m away? (in air at standard T and P) what is the length, in m, of the pulse?

I realise that light has a wavelength and when you pulse it that wavelength shouldn't change. Really I just don't know the right equations to solve the problem

I'm struggling with this pre-lab homework as I'm taking a physics paper out of interest and cannot find any concise or simple enough explanations of the answer in the readings I've been given or on the internet. Any help would be greatly appreciated.

Cheers
If you throw a javelin that is 2m long at 20m/s horizontally where there is no gravity (so it keeps flying straight horizontally), how long does it take for it to reach 20m downrange? How long does it take it to pass through from tip-to-end the 100m mark?
 
  • #4
Do you know what light speed is? Do you know what "1 ms" is? Do you know what a "rate equation" is?
ms is millisecond? not meters per second. so the period of the pulse, or the time that the light source was turned on then off again is 1ms, that is my understanding @berkeman if it were a pulse traveling at 1 m/s it would take 20s to cross 20m. but from my understanding the pulse's velocity is not given.
 
  • #5
CWatters
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Is this homework or an exam question? If homework look up the velocity.
 
  • #6
Is this homework or an exam question? If homework look up the velocity.
Homework, do you mean search on the forums for velocity?
 
  • #7
jbriggs444
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Homework, do you mean search on the forums for velocity?
You could Google for "what is the speed of light"
 
  • #8
CWatters
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+1

If it's homework they mean you to look up the speed of light.
If it's an exam question they either give it to you or you are meant to know what I is.
 
  • #9
+1

If it's homework they mean you to look up the speed of light.
If it's an exam question they either give it to you or you are meant to know what I is.
I realise that this is a light problem. but I don't see how I would use the speed of light? with what formula?
 
  • #10
berkeman
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how long will the pulse take to reach the other side of the room 20m away?
I realise that this is a light problem. but I don't see how I would use the speed of light? with what formula?
What is the equation that relates distance, speed and time? Be sure to use units to help you get the equation correct.
 
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  • #11
PeterO
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What is the equation that relates distance, speed and time? Be sure to use units to help you get the equation correct.
There are many modern water features/fountains/displays that have a stream of water turned on, then off a short time late (longer than a millisecond. The water streams I have seen seem to be travelling at about 3 m/s.
Suppose such a stream of water could move horizontally across a 6, room without falling to the floor. Lets say the speed of the water is 3 m/s, and the water was turned on for 0.1 seconds (one tenth of a second)
How long would the cylinder of water be (its diameter would be determined by the opening through which it passed when created), and how long would the cylinder of water take to cross the room?

Your light problem is very similar to this, it is just the light travels much faster.

This video shows the sort of fountain I was referring to - sometimes the stream is continuous, but I am referring to the pulses.
 
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  • #12
What is the equation that relates distance, speed and time? Be sure to use units to help you get the equation correct.
I think I figured it out. Handed it in yesterday. Used the equation for velocity, d/t. I was over complicating it a bit I think. Thanks a bunch for your help!
 

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