Skip to main content
Physics

Return to Question

Post Timeline

Commonmark migration
Source Link
Community Bot
Community Bot
  • 1

XKCD usually has solid (and often contemporary) science behind it. Lightning Difference, #2027 one says:

Q: What’s that trick for telling how many miles away lightning is?

 

A: Just count the seconds between the visible flash and the radio wave burst, then multiply by 5 billion.

Usually it's lightning versus thunder, and you divide the time by 5 (or thereabouts) to get the distance in miles.

Here though, light time for 1 mile (about 1600 meters) would be about 5.3E-06 seconds, and if the difference between the visible light flash and the radio burst were one five-billionths of a second (2E-10 seconds), that suggests a velocity difference of about 38 ppm.

What is the physics behind that 38 ppm difference?

enter image description here

XKCD usually has solid (and often contemporary) science behind it. Lightning Difference, #2027 one says:

Q: What’s that trick for telling how many miles away lightning is?

 

A: Just count the seconds between the visible flash and the radio wave burst, then multiply by 5 billion.

Usually it's lightning versus thunder, and you divide the time by 5 (or thereabouts) to get the distance in miles.

Here though, light time for 1 mile (about 1600 meters) would be about 5.3E-06 seconds, and if the difference between the visible light flash and the radio burst were one five-billionths of a second (2E-10 seconds), that suggests a velocity difference of about 38 ppm.

What is the physics behind that 38 ppm difference?

enter image description here

XKCD usually has solid (and often contemporary) science behind it. Lightning Difference, #2027 one says:

Q: What’s that trick for telling how many miles away lightning is?

A: Just count the seconds between the visible flash and the radio wave burst, then multiply by 5 billion.

Usually it's lightning versus thunder, and you divide the time by 5 (or thereabouts) to get the distance in miles.

Here though, light time for 1 mile (about 1600 meters) would be about 5.3E-06 seconds, and if the difference between the visible light flash and the radio burst were one five-billionths of a second (2E-10 seconds), that suggests a velocity difference of about 38 ppm.

What is the physics behind that 38 ppm difference?

enter image description here

Tweeted twitter.com/StackPhysics/status/1026302682246434819
Question Protected by Qmechanic♦
edited tags
Link
Qmechanic ♦
Qmechanic ♦
  • 227.3k
  • 52
  • 651
  • 2.7k
Source Link
uhoh
uhoh
  • 6.3k
  • 5
  • 33
  • 106

What's the physics behind XKCD #2027 (time between lightning flash and radio wave burst)?

XKCD usually has solid (and often contemporary) science behind it. Lightning Difference, #2027 one says:

Q: What’s that trick for telling how many miles away lightning is?

A: Just count the seconds between the visible flash and the radio wave burst, then multiply by 5 billion.

Usually it's lightning versus thunder, and you divide the time by 5 (or thereabouts) to get the distance in miles.

Here though, light time for 1 mile (about 1600 meters) would be about 5.3E-06 seconds, and if the difference between the visible light flash and the radio burst were one five-billionths of a second (2E-10 seconds), that suggests a velocity difference of about 38 ppm.

What is the physics behind that 38 ppm difference?

enter image description here