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Question about electromagnetic radiation and distance...?

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  • 01-01-2023 1:20pm
    #1
    Registered Users Posts: 3,395 ✭✭✭


    Bear with me on the science of my explanation...that's partly why I'm asking the question. ☺️

    Let's say that light takes 1.5 seconds to reach the moon.

    I switch on a torch and 1.5 seconds later, the man on the moon sees it. (Yes, it's a big torch.)

    If I switch on and off my torch within 1.5 seconds, the man on the moon never sees the light beaming from my torch, or does he?

    Furthermore, if I want to make contact with life many light-years from here, must my torch (or radio signal) remain on until it reaches other life, or does a packet of energy continue forever (albeit, increasingly faintly) if the source of that energy back on earth is turned off?

    Thanks.

    D.



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  • Moderators, Science, Health & Environment Moderators Posts: 1,847 Mod ✭✭✭✭Michael Collins


    If I switch on and off my torch within 1.5 seconds, the man on the moon never sees the light beaming from my torch, or does he?

    He will see it, for as long as you had the light turned on. It's just delayed, that's all.

    This is the reason it's called electromagnetic 'radiation'. The light is radiated, i.e. leaves, the torch. Turning off the torch after will have no effect on the light that's already left.

    Turning off the torch also will not affect the received brightness. That will be the same as it ever was. The farther the light travels, it may get dimmer as the wavefront tends to spread out, but this happens regardless of what's happening with the original source.

    You can probably answer your last question yourself now!



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