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Suppose, you and your friend are present in a pitch black, perfect vacuum side by side. Your friend has a flashlight which he flashes in front of him. Will you be able to see/perceive the light or will it remain pitch black for you?

Qmechanic
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user336972
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  • I think this question is undefined. What's the definition of a perfect vacuum? Why do you think that light could travel only on a "imperfect" vacuum and, if so, what component of that "imperfect" vacuum do you think would be necessary for light to travel? – Miyase Jun 12 '22 at 11:42
  • Why do you think a vacuum is black? I would rather think vacuum is transparent. – Thomas Fritsch Jun 12 '22 at 11:56
  • Depends on the source of light. If you're talking a laser pointer, then no. A normal flash light maybe. If the EM waves hits the other person then yes. It depends entirely on the source and what EM wave it produces, and how good it is at containing the em wave. – jensen paull Jun 12 '22 at 13:39
  • Possible duplicates: https://physics.stackexchange.com/q/546845/2451 and links therein. – Qmechanic Jun 12 '22 at 13:55

2 Answers2

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First of all, there is no such thing as a perfect vacuum. Even if it were devoid of any matter, quantum vacuum fluctuations would spontaneously result in particle-antiparticle pairs. Moreover, a vacuum has no colour. It's not pitch black but transparent as nothing absorbs or reflects the light. If your friend were to shine a flashlight in a perfect vacuum, you wouldn't be able to see the path the light takes because of the lack of particles to reflect and disperse the light. However, the path of light would be visible in a dusty room or any place where the particles are large enough to disperse the light rays.

Mechanic
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Vacuum is the absence of matter not the absence of light. Light does not require matter to propagate through, in fact it travels faster without matter in the way to interact with it.

The photons (light particles) are emitted from the light bulb in all directions. If photons manage to reach from the person holding the flashlight to his friend he will perceive the light.

If the flashlight has a casing that obscures some of the isotropic emission, and the flashlight is directed away from the other person, light would simply not reach the other person and he would not perceive the light.

If they were floating in a gas (not in a vacuum) the friend might see the glare from the flashlight due to reflections from the gas particles.

Bottom line is - photons need to reach the other persons eyes in order for him to perceive his friends light.

Ben
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