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How Does Light Travel Through Space?



Every point of light you see in the sky is an entire world sending out energy in the form of light. Your ability to perceive this light even across such vast distances says a lot about not just the nature of light, but how powerful the sources of that light really are. Here’s what it takes for light to travel through space.



1. Photons may be particles, but light travels as a wave.
The double-slit experiment is a famous one for good reason: it demonstrated that light can behave both as individual particles and as a wave. When light travels through space, it propagates as a wave, but in a different way than other types of waves. Sound waves, for example, need a medium to interact with, and since there’s not enough densely packed matter in space for sound to travel on, soundwaves don’t carry through a vacuum. Light waves, on the other hand, don’t need anything to travel through, so they can move quite easily through space.

2. There’s nothing for the light to interact with, so it travels on and on.
Since the light doesn’t need a medium to travel with, it isn’t hindered in any way and will keep on going. It won’t dissipate, and it will continue to expand out forever.



3. Stars send out light in every direction.
If light doesn’t dissipate, why do some stars appear dimmer? This has to do with the amount of light we receive from the source. The light from a distant star is being sent out in all directions in a spherical configuration, and it will fill the entire space afforded to it. This means that for a star that’s very distant, only a tiny sliver of the light being sent out actually reaches our eyes. The light itself hasn’t dimmed on the way; the amount we receive has reduced.
 
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Jun 1, 2020
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Nice.

Many like to refer to light packets, where photons travel in great numbers, even when diminished by distance by the inverse square law.

But some of those photons do become absorbed or scattered. The dark regions in your nice galaxy image are very likely due to photons that were absorbed or scattered by a cloud of gas and dust. This dimming effect, along with specific bands of scattering, provide astronomers with very useful information of these nebulae.
 

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Every point of light you see in the sky is an entire world sending out energy in the form of light. Your ability to perceive this light even across such vast distances says a lot about not just the nature of light, but how powerful the sources of that light really are. Here’s what it takes for light to travel through space.



1. Photons may be particles, but light travels as a wave.
The double-slit experiment is a famous one for good reason: it demonstrated that light can behave both as individual particles and as a wave. When light travels through space, it propagates as a wave, but in a different way than other types of waves. Sound waves, for example, need a medium to interact with, and since there’s not enough densely packed matter in space for sound to travel on, soundwaves don’t carry through a vacuum. Light waves, on the other hand, don’t need anything to travel through, so they can move quite easily through space.

2. There’s nothing for the light to interact with, so it travels on and on.
Since the light doesn’t need a medium to travel with, it isn’t hindered in any way and will keep on going. It won’t dissipate, and it will continue to expand out forever.



3. Stars send out light in every direction.
If light doesn’t dissipate, why do some stars appear dimmer? This has to do with the amount of light we receive from the source. The light from a distant star is being sent out in all directions in a spherical configuration, and it will fill the entire space afforded to it. This means that for a star that’s very distant, only a tiny sliver of the light being sent out actually reaches our eyes. The light itself hasn’t dimmed on the way; the amount we receive has reduced.
Yes, think of a prism.
 

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